Vaillant aquaPLUS Fault codes & diagnostics

94 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural GasLPG 28-32 kW 2003-2009 Discontinued

Replaced by: ecoTEC plus 937

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 34 High 7 Medium 52 Low
Just want the most common Vaillant aquaPLUS faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 94 documented codes

Tap any card for details

d.00

Your boiler is currently running at a lower power setting than its maximum capacity, which is a normal operational adjustment rather than a breakdown.

Low DIY-safe

What to check first

  1. Check that your radiators are turned up
  2. Ensure your thermostat is calling for heat
  3. Monitor the boiler as it will likely exit this status automatically once demand increases

Related codes

Full explanation for d.00
d.01

Your boiler is continuing to circulate water after heating has finished to safely cool down the internal components.

Low DIY-safe

What to check first

  1. Wait for the timer to finish as this is normal operation
  2. Check that your room thermostat is not set too high
  3. Ensure radiator valves are open to allow water flow

Related codes

Full explanation for d.01
d.02

Your boiler is temporarily pausing its burner to prevent it from switching on and off too frequently, which helps protect the components from wearing out.

Low DIY-safe

What to check first

  1. Wait 15-30 minutes for the countdown to end and the boiler to restart automatically
  2. Ensure all radiator valves are fully open to allow heat to dissipate
  3. Check that the thermostat is calling for heat

Related codes

Full explanation for d.02
d.03

Your boiler is currently showing you exactly how hot the water inside the storage tank is, rather than indicating a fault.

Low DIY-safe

What to check first

  1. Press the 'i' button to exit the diagnostic information mode
  2. Wait for the display to automatically return to the standard pressure or temperature view
  3. No further action is required as this is a status information code, not an error

Related codes

Full explanation for d.03
d.04

Your boiler is currently showing you the live temperature of the water inside your storage tank rather than an error code.

Low DIY-safe

What to check first

  1. Press the 'i' button to exit the diagnostic mode
  2. Wait for the display to revert to the standard pressure or temperature view
  3. If the screen is stuck, briefly turn the boiler off and back on at the fused spur switch

Related codes

Full explanation for d.04
d.05

Your boiler is displaying the current temperature it is aiming for as it heats your water, which is a normal status message rather than a breakdown.

Low DIY-safe

What to check first

  1. Check if your heating or hot water is working as normal
  2. Adjust your thermostat or boiler temperature dial if you wish to change this setting
  3. Monitor the display to see if it changes to a different status code during operation

Related codes

Full explanation for d.05
d.06

Your boiler is displaying the temperature setting you have selected for your hot water, rather than an actual error.

Low DIY-safe

What to check first

  1. No action is required as this is an information code, not a fault.
  2. Adjust your hot water control knob if you wish to change the target temperature.
  3. Wait for the display to return to the standard pressure or temperature reading.

Related codes

Full explanation for d.06
d.08

Your boiler has received a signal from your thermostat or external programmer to start heating your home.

Low DIY-safe

What to check first

  1. Check your room thermostat to see if it is turned up higher than the current room temperature.
  2. Verify if your central heating timer or programmer is currently set to an 'on' period.
  3. No action is required if you actually want the heating to be running, as this is a status message rather than a fault.

Related codes

Full explanation for d.08
d.10

Your boiler is currently reporting whether the internal central heating pump is switched on or off.

Low DIY-safe

What to check first

  1. Check if your thermostat or programmer is calling for heat
  2. Monitor the display to see if the value changes from 0 (off) to 1 (on) when heating is active
  3. No action is required as this is an information status code, not an error

Related codes

Full explanation for d.10
d.11

Your boiler is struggling to communicate with or power the external pump that circulates hot water around your heating system.

High Engineer only £180-350

Parts commonly replaced

  • External Circulation Pump
  • PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

Full explanation for d.11
Showing 11–20 of 94
d.12

Your boiler is currently reporting the operational status of the secondary hot water pump, which is often a normal part of the start-up sequence but can indicate a communication delay if it stays on the screen.

Low DIY-safe £0-120

What to check first

  1. Check if the hot water is still running as normal, as this is often an information code rather than a lockout fault
  2. Switch the boiler off and back on at the fused spur switch to refresh the system status
  3. Ensure there is no air trapped in the system by checking that your radiator valves are open

Parts commonly replaced

  • No parts usually required
  • Secondary DHW pump
  • PCB

Related codes

Full explanation for d.12
d.13

Your boiler is reporting a communication issue or a power fault with the external pump that circulates hot water around your taps.

Low Engineer only £120-280

Parts commonly replaced

  • External DHW circulation pump
  • Wiring harness
  • Control PCB

Related codes

Full explanation for d.13
d.21

Your boiler is having trouble detecting if the flame has successfully lit, which prevents it from running safely.

High Engineer only £120-280

Parts commonly replaced

  • Ionisation electrode
  • Ignition lead
  • PCB

Related codes

Full explanation for d.21
d.22

Your boiler has detected that a hot water tap is currently turned on or there is a demand for hot water.

Low DIY-safe

What to check first

  1. Check all hot water taps are fully closed
  2. Ensure any showers or mixer taps are not leaking or dripping
  3. Wait for the boiler to finish providing hot water, as this is a status code rather than a fault

Related codes

Full explanation for d.22
d.23

Your boiler is currently set to summer mode, which means it will provide hot water but your central heating is switched off.

Low DIY-safe

What to check first

  1. Locate the left-hand control knob on the boiler fascia with the radiator icon.
  2. Turn the knob clockwise to your desired heating temperature setting.
  3. The 'd.23' status should change as the boiler switches back to winter mode.

Related codes

Full explanation for d.23
d.24

Your boiler is struggling to push out exhaust gases safely, which is preventing the system from firing up properly.

High Engineer only £120-£280

Parts commonly replaced

  • Air Pressure Switch
  • Venturi Tube
  • Fan Assembly

Related codes

Full explanation for d.24
d.25

Your boiler is currently heating the internal water storage tank to ensure you have hot water ready to use.

Low DIY-safe

What to check first

  1. No action is required as this is a normal status message rather than an error.
  2. Wait for the boiler to finish the heating cycle.
  3. Check that your hot water temperature dial is set to your preferred level.

Related codes

Full explanation for d.25
d.30

Your boiler's control system is checking whether the gas valve is open or closed, which is part of its normal safety process.

Low DIY-safe £0-120

What to check first

  1. No action required; this is an information code, not a fault
  2. Monitor the display to see if it changes to a standard temperature reading
  3. If the boiler is not firing, try a simple reset

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for d.30
d.33

Your boiler's fan is not reaching the speed required to safely clear exhaust fumes, preventing the system from starting up.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Air pressure switch

Related codes

Full explanation for d.33
d.34

Your boiler is checking the current speed of the internal fan to ensure it is venting exhaust gases correctly.

Low DIY-safe £0-120

What to check first

  1. No action is typically required as this is an information code rather than a permanent error.
  2. If the boiler is stuck on this screen, try pressing the 'Reset' button (flame icon).
  3. Ensure there are no obvious obstructions to the flue terminal on the outside of your property.

Parts commonly replaced

  • None (Status code)
  • Fan
  • Printed Circuit Board (PCB)

Related codes

Full explanation for d.34
Showing 21–30 of 94
d.35

Your boiler is currently switching its internal components to test whether it should be sending heat to your radiators or your hot water tank.

Low DIY-safe

What to check first

  1. Observe the boiler for a few minutes; this is usually a normal diagnostic check rather than a fault.
  2. If the code stays on screen permanently, try resetting the boiler using the button marked with a flame or 'Reset'.
  3. Check if your heating and hot water are still working as normal.

Related codes

Full explanation for d.35
d.37

Your boiler is struggling to control the intensity of its flame, which stops it from heating your water and radiators efficiently.

High Engineer only £180-£350

Parts commonly replaced

  • Gas valve
  • Gas valve modulator coil
  • Main PCB

Related codes

Full explanation for d.37
d.40

Your boiler is currently displaying the live temperature of the water flowing through the system rather than a fault code.

Low DIY-safe

What to check first

  1. No action is required as this is an informational status code.
  2. Press the 'i' button to exit the information menu if you wish to see the system pressure.
  3. If the boiler is not firing, check that your thermostat is turned up and calling for heat.

Related codes

Full explanation for d.40
d.41

Your boiler is simply displaying the current temperature of the water returning from your radiators to the unit, rather than indicating a breakdown or fault.

Low DIY-safe

What to check first

  1. Press the 'i' button to exit the information menu
  2. Monitor the screen to see if it reverts to the standard pressure or temperature display
  3. No action is required as this is an informative status code, not an error

Related codes

Full explanation for d.41
d.50

Your boiler has reached its maximum allowed internal temperature and is pausing operation to prevent damage.

Medium DIY-safe £80-200

What to check first

  1. Check that all radiator valves (TRVs) are fully open
  2. Ensure the central heating system pressure is between 1.0 and 1.5 bar
  3. Turn the heating control knob down slightly and reset the boiler

Parts commonly replaced

  • NTC Thermistor
  • Circulation Pump
  • Main Heat Exchanger (cleaning)

Related codes

Full explanation for d.50
d.60

Your boiler has shut down because it triggered its safety protection against overheating too many times and requires a professional investigation to find the underlying cause.

High Engineer only £120-450

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Temperature sensors (NTC thermistors)
  • Diverter valve

Related codes

Full explanation for d.60
d.61

Your boiler is displaying a history log showing it has recently experienced one or more system shutdowns due to a persistent fault.

Medium DIY-safe £0-250

What to check first

  1. Press the reset button (flame symbol with a cross) to clear any active lockout
  2. Check that your credit meter or gas supply is active
  3. Monitor the boiler for specific 'F' error codes that follow the reset

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for d.61
d.64

Your boiler is taking longer than usual to light, which may indicate a slight delay in the ignition process or a sensor beginning to age.

Low Engineer only £80-200

Parts commonly replaced

  • Ignition electrode
  • Flame sense electrode
  • Gas valve

Related codes

Full explanation for d.64
F.00

Your boiler has lost its connection to the temperature sensor that monitors water flowing through the system, preventing it from heating up safely.

High Engineer only £90-160

Parts commonly replaced

  • Flow NTC Thermistor
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for F.00
F.01

Your boiler has stopped working because one of its internal temperature sensors is unable to send a signal, meaning the system cannot tell if it is safe to heat the water.

High Engineer only £90-180

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness
  • PCB (Printed Circuit Board)

Related codes

Full explanation for F.01
Showing 31–40 of 94
F.02

Your boiler's hot water sensor has stopped communicating with the main control board, meaning the system can no longer tell how hot your water is.

High Engineer only £90-160

Parts commonly replaced

  • DHW NTC Temperature Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for F.02
F.03

Your boiler has lost communication with the temperature sensor in its internal hot water storage tank, meaning it cannot safely monitor or heat your hot water.

High Engineer only £90-180

Parts commonly replaced

  • Storage Tank NTC Sensor
  • NTC Wiring Harness

Related codes

Full explanation for F.03
F.10

Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water flowing out to your taps and radiators.

High Engineer only £95-170

Parts commonly replaced

  • Flow NTC thermistor
  • NTC sensor wiring loom

Related codes

Full explanation for F.10
F.11

Your boiler has detected an electrical fault with the sensor that monitors water returning from your radiators, often caused by a faulty part or damaged wiring.

High Engineer only £90-180

Parts commonly replaced

  • Return NTC thermistor (sensor)
  • Wiring harness / Connector plug

Related codes

Full explanation for F.11
F.12

Your boiler has detected a faulty sensor that monitors your hot water temperature, meaning it cannot safely heat water for your taps.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC sensor
  • Wiring harness

Related codes

Full explanation for F.12
F.13

Your boiler's hot water storage tank sensor has developed an electrical fault, meaning the system can no longer monitor or control the temperature of your hot water.

Medium Engineer only £100-180

Parts commonly replaced

  • Storage tank NTC sensor
  • Wiring harness connector

Related codes

Full explanation for F.13
F.20

Your boiler has switched itself off because it has reached an internal temperature that is too high, often caused by a blockage or poor water flow.

High DIY-safe £120-£350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Press the reset button (flame symbol with a cross) to restart the boiler

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • NTC Temperature Sensor

Related codes

Full explanation for F.20
F.22

Your boiler has detected that there is not enough water pressure or flow to operate safely, causing it to shut down to prevent overheating.

High DIY-safe £100-£350

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. Locate the filling loop (usually two silver braided hoses) underneath the boiler.
  3. Slowly open the valves to let water into the system until the gauge reaches 1.5 bar, then ensure they are closed tightly.
  4. Press the reset button (indicated by a flame with a cross through it) to restart the boiler.

Parts commonly replaced

  • Circulation Pump
  • Pressure Sensor
  • Expansion Vessel

Related codes

Full explanation for F.22
F.23

Your boiler has detected that the water isn't moving through the system quickly enough, causing it to heat up too fast and shut down to prevent damage.

High Engineer only £180-350

Parts commonly replaced

  • Circulation Pump
  • NTC Thermistor
  • Pump Cable

Related codes

Full explanation for F.23
F.24

Your boiler is heating up too quickly because the water inside isn't moving fast enough, usually due to trapped air or a struggling pump.

High DIY-safe £150-£350

What to check first

  1. Check that your radiator valves are fully open
  2. Bleed all radiators to remove trapped air from the system
  3. Check the boiler pressure is between 1.0 and 1.5 bar and top up if necessary

Parts commonly replaced

  • Circulation Pump
  • Flow/Return NTC Sensor
  • Automatic Air Vent

Related codes

Full explanation for F.24
Showing 41–50 of 94
F.27

Your boiler has detected a false flame signal or a communication error between the gas valve and the control board, preventing it from firing up safely.

High Engineer only £180-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Gas Valve
  • Ignition Electrodes

Related codes

Full explanation for F.27
F.28

Your boiler is failing to light the gas flame despite several attempts, meaning you currently have no heating or hot water.

High DIY-safe £120-280

What to check first

  1. Check that your gas emergency control valve is in the 'on' position.
  2. Verify if other gas appliances in your home, such as a hob, are receiving gas.
  3. If you have a pre-payment meter, check that you have active credit.
  4. Press the 'Reset' button (indicated by a flame with a line through it) for five seconds.

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Spark Leads
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F.28
F.29

Your boiler has lost its flame while running and cannot manage to relight itself, likely due to a problem with the gas supply.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances in your home, like a hob, are working correctly.
  2. Ensure your gas meter has credit if you use a prepay system.
  3. Ensure the gas isolation valve under the boiler is fully open.
  4. Press the 'Reset' button (flame symbol with a cross through it) to restart the ignition sequence.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Condensate Trap or Pipe
  • Gas Meter Regulator

Related codes

Full explanation for F.29
F.32

Your boiler has stopped working because the fan is spinning at the wrong speed, causing the system to lock as a safety precaution.

High Engineer only £220-£450

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F.32
F.33

Your boiler has stopped working because it cannot confirm that waste gases are being safely vented out of the flue.

High Engineer only £120-£350

Parts commonly replaced

  • Air pressure switch
  • Fan assembly
  • Venturi tube

Related codes

Full explanation for F.33
F.37

Your boiler has stopped working because the fan that helps remove exhaust gases is not spinning at the correct speed.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • PCB (Main circuit board)

Related codes

Full explanation for F.37
F.41

Your boiler has lost its settings and doesn't know whether it should be burning natural gas or LPG, causing it to stop working for safety reasons.

High Engineer only £80-250

Parts commonly replaced

  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for F.41
F.42

Your boiler has detected an internal electrical wiring issue or a faulty connection that prevents it from running safely.

High Engineer only £120-£250

Parts commonly replaced

  • Wiring harness
  • Main PCB
  • Internal cable loom

Related codes

Full explanation for F.42
F.43

Your boiler has stopped working because its internal electrical wiring is either loose, damaged, or incorrectly configured.

High Engineer only £120-£280

Parts commonly replaced

  • Wiring harness
  • Main PCB
  • Internal cable loom

Related codes

Full explanation for F.43
F.60

Your boiler's internal computer has developed a fault and can no longer communicate with the gas valve or control the heating process.

High Engineer only £280-550

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Gas Valve Stepper Motor
  • Wiring Loom

Related codes

Full explanation for F.60
Showing 51–60 of 94
F.61

Your boiler's main control computer or the gas valve connection has suffered an internal electrical failure, preventing the system from operating safely.

High Engineer only £280-550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Gas Valve
  • Wiring Harness

Related codes

Full explanation for F.61
F.62

Your boiler is still detecting a flame even though it has commanded the gas to turn off.

Emergency Engineer only £180-450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Ionisation Electrode

Related codes

Full explanation for F.62
F.63

Your boiler’s internal computer has encountered a communication error or electrical fault that is preventing it from running safely.

High Engineer only £250-£550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Internal wiring harness

Related codes

Full explanation for F.63
F.64

Your boiler has detected a communication error between its internal sensors and the main control board, preventing it from operating safely.

High Engineer only £140-£450

Parts commonly replaced

  • NTC Temperature Sensor
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for F.64
F.65

Your boiler's internal control board is overheating, causing the system to shut down to prevent damage.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

Full explanation for F.65
F.67

Your boiler’s internal computer has detected a communication error or a hardware failure within its control system, preventing the boiler from starting.

High Engineer only £280-550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Operator Interface Control Box

Related codes

Full explanation for F.67
P.11

Your boiler is currently running a setup program to test its gas pressure and is not in its normal operating mode.

Low DIY-safe

What to check first

  1. Wait for the program to finish automatically
  2. Press the 'i' button to monitor progress
  3. If it persists, turn the boiler off and on again to reset it

Related codes

Full explanation for P.11
P.12

Your boiler is undergoing a temporary waiting process while it determines if the water pressure is high enough to run safely.

Low DIY-safe £0-60

What to check first

  1. Check the pressure gauge on the front of the boiler.
  2. If the pressure is below 1.0 bar, use the filling loop handles to top it up to 1.5 bar.
  3. Restart the boiler once the pressure is correct.

Parts commonly replaced

  • None (Pressure adjustment only)
  • Filling loop

Related codes

Full explanation for P.12
P.13

Your boiler is currently running a special setup routine to calibrate the gas pressure for Liquid Petroleum Gas (LPG) operation.

Low Engineer only £80-120

Parts commonly replaced

  • None - setting adjustment only
  • Gas valve

Related codes

Full explanation for P.13
Showing 61–70 of 94
S.00

Your boiler is working correctly but is currently resting because it hasn't been told to turn on by your thermostat or timer.

Low DIY-safe

What to check first

  1. Check that your room thermostat is turned up higher than the current room temperature
  2. Ensure your programmer or heating timer is set to an 'ON' period
  3. Check if the batteries in your wireless thermostat need replacing

Related codes

Full explanation for S.00
S.01

Your boiler is currently preparing to provide heating and is going through its normal start-up sequence.

Low DIY-safe

What to check first

  1. No action is required as this is a status message rather than a fault.
  2. Ensure your thermostat is calling for heat.
  3. Wait a few minutes for the boiler to complete its cycle and begin heating your radiators.

Related codes

Full explanation for S.01
S.02

Your boiler is currently warming up your radiators as part of its normal heating process.

Low DIY-safe

What to check first

  1. Check your thermostat or programmer settings to see if heating is requested
  2. Monitor the display to see if the code changes to S.04 once the boiler fires up
  3. No action is required if your radiators are getting warm as expected

Related codes

Full explanation for S.02
S.03

Your boiler is currently trying to light the flame to start your central heating.

Low DIY-safe

What to check first

  1. Check if your thermostat is calling for heat
  2. Wait a few minutes to see if the boiler transitions to a run code
  3. Ensure your gas supply is turned on if the boiler fails to light

Related codes

Full explanation for S.03
S.05

Your boiler is simply cooling itself down after a period of heating, which is a normal part of its operation.

Low DIY-safe

What to check first

  1. Wait for a few minutes as this status code usually clears automatically once the boiler has cooled.
  2. Ensure your radiator valves are open to allow heat to dissipate.
  3. Check that your room thermostat is not set significantly higher than the current room temperature.

Related codes

Full explanation for S.05
S.06

Your boiler is continuing to run its internal fan for a short period to clear out heat or gases after providing heating.

Low DIY-safe

What to check first

  1. No action is required as this is a normal status message rather than a breakdown code.
  2. Wait for the fan to stop automatically once the unit has cooled down.
  3. If the fan never stops, try resetting the boiler using the button marked with a flame symbol.

Related codes

Full explanation for S.06
S.07

Your boiler is briefly keeping the pump running after heating has turned off to help the system cool down safely.

Low DIY-safe

What to check first

  1. Check that all your radiator valves (TRVs) are not fully turned off at the same time
  2. Ensure the central heating system is pressurized between 1.0 and 1.5 bar
  3. Monitor the boiler to see if it moves to a standard 'S.0' standby code after a few minutes

Related codes

Full explanation for S.07
S.08

Your boiler has reached its required temperature and is taking a short, planned break to prevent it from switching on and off too frequently.

Low DIY-safe

What to check first

  1. Wait 15 to 20 minutes for the timer to finish
  2. Check that your room thermostat is turned up
  3. Ensure your radiator valves are open to allow heat to dissipate

Related codes

Full explanation for S.08
Showing 71–80 of 94
S.15

Your boiler is briefly continuing to run the fan and pump after you have finished using hot water to safely cool down the internal components.

Low DIY-safe

What to check first

  1. No action is required as this is a normal operating status and not a fault.
  2. Wait for the 'S' code to disappear once the cooling cycle is complete.
  3. Ensure your taps are fully turned off.

Related codes

Full explanation for S.15
S.16

Your boiler's fan is staying on for longer than usual after you have used hot water to help cool the internal components down.

Low DIY-safe £0-120

What to check first

  1. Check if your hot water taps are fully closed
  2. Monitor the display to see if the code clears automatically after a few minutes
  3. Ensure the boiler's casing ventilation is not obstructed

Parts commonly replaced

  • None (usually a normal status code)
  • Temperature Sensor (NTC)
  • Fan

Related codes

Full explanation for S.16
S.17

Your boiler is temporarily running the pump to move excess heat away from the hot water heat exchanger after use.

Low DIY-safe

What to check first

  1. Wait up to 20 minutes for the status code to change naturally
  2. Ensure radiators are not all turned off at their valves
  3. Check that the boiler pressure gauge is between 1 and 1.5 bar

Related codes

Full explanation for S.17
S.21

Your boiler is currently busy heating up the internal hot water storage tank so you have plenty of hot water ready for use.

Low DIY-safe

What to check first

  1. No action is required as this is a normal status message, not a fault.
  2. Wait approximately 15-20 minutes for the 'S' code to clear once the tank is hot.
  3. Check that your hot water temperature dial is set to your preferred level.

Related codes

Full explanation for S.21
Showing 81–90 of 94
S.24

Your boiler is currently busy reheating the built-in hot water storage tank so you have plenty of hot water ready to use.

Low DIY-safe

What to check first

  1. Wait approximately 15-20 minutes for the tank to finish refilling
  2. Check that your hot water temperature dial is set to your desired level
  3. Ensure no taps are left running which would prolong the filling process

Related codes

Full explanation for S.24
S.25

Your boiler is currently cooling down its internal components after heating up your hot water tank.

Low DIY-safe

What to check first

  1. No action is required as this is a normal operational status message, not a fault.
  2. Wait for the 'S' code to disappear once the cooling cycle finishes.
  3. Ensure your room thermostat is calling for heat if you want the radiators to come on next.

Related codes

Full explanation for S.25
S.26

Your boiler is currently running the internal fan to move heat from the burner into your hot water storage tank.

Low DIY-safe

What to check first

  1. Check that your hot water controls are set to the desired temperature
  2. Wait up to 20 minutes for the 'store charging' process to complete naturally
  3. Monitor the display to see if it transitions to a different code or back to a standard temperature display

Related codes

Full explanation for S.26
S.27

Your boiler is briefly running the pump after heating your hot water cylinder to move excess heat away and protect the internal components.

Low DIY-safe £0-120

What to check first

  1. Check that at least one radiator valve (the bypass radiator) is fully open
  2. Ensure that the boiler's water pressure is between 1.0 and 1.5 bar
  3. Wait up to 10 minutes for the status code to clear automatically as it is often part of normal operation

Parts commonly replaced

  • None (Normal operation)
  • Pump
  • NTC Temperature Sensor

Related codes

Full explanation for S.27
S.28

Your boiler is taking a temporary break to prevent the system from turning on and off too frequently while it heats the internal hot water storage tank.

Low DIY-safe

What to check first

  1. Check if any hot water taps are slightly open or dripping
  2. Wait 15-20 minutes for the timer to count down and the boiler to restart automatically
  3. Ensure your thermostat is set to the desired temperature

Related codes

Full explanation for S.28
S.30

Your boiler is working fine, but your thermostat or programmer is not telling it to turn the heating on.

Low DIY-safe £0-120

What to check first

  1. Check if your room thermostat is set higher than the current room temperature.
  2. Replace the batteries in your wireless thermostat or programmer.
  3. Ensure your heating timer or external controller is set to 'ON' or 'AUTO'.

Parts commonly replaced

  • Thermostat batteries
  • Room thermostat
  • External programmer

Related codes

Full explanation for S.30
S.31

Your boiler is currently set to summer mode, which means it will provide hot water but has been told not to turn on the central heating.

Low DIY-safe

What to check first

  1. Locate the central heating control knob (the one with the radiator symbol) on the boiler's front panel.
  2. Turn the knob clockwise away from the 'off' or sun icon position to your desired temperature setting.
  3. Ensure your room thermostat or external heating controls are also calling for heat.

Related codes

Full explanation for S.31
S.32

Your boiler has paused because the internal fan is not spinning at the correct speed to safely clear waste gases.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Harness wiring

Related codes

Full explanation for S.32
S.33

Your boiler is struggling to breathe properly because it has detected a problem with the air being pulled in or the exhaust fumes being pushed out.

High Engineer only £120-350

Parts commonly replaced

  • Fan
  • Air pressure switch
  • Flue terminal
  • Venturi tube

Related codes

Full explanation for S.33
S.34

Your boiler has detected very cold temperatures and has automatically turned on its built-in safety mode to prevent the internal pipes from freezing.

Low DIY-safe

What to check first

  1. Ensure your main heating controls or thermostat are turned up slightly to help warm the system
  2. Check that any radiator valves in cold rooms are open
  3. Ensure there is power to the boiler so the pump can circulate the warm water

Related codes

Full explanation for S.34
Showing 91–94 of 94
S.36

Your boiler is working fine, but it is currently waiting for a signal from your room thermostat or programmer to tell it to turn the heating on.

Low DIY-safe £0-120

What to check first

  1. Check that your room thermostat is turned up higher than the current room temperature.
  2. Ensure your central heating programmer or timer is set to 'ON' or 'AUTO'.
  3. Replace the batteries in your wireless thermostat if the screen is blank or low.

Parts commonly replaced

  • Thermostat batteries
  • External room thermostat
  • Heating programmer

Related codes

Full explanation for S.36
S.39

Your boiler has paused because it detected that the water temperature going into your underfloor heating is too high, preventing damage to your floors.

Medium DIY-safe £90-180

What to check first

  1. Check if your underfloor heating pump is running
  2. Ensure any manual mixing valves for the underfloor heating are not set too high
  3. Gently bleed air from the underfloor heating manifold to improve circulation

Parts commonly replaced

  • Overheat clip-on thermostat
  • Underfloor heating mixing valve
  • Circulation pump

Related codes

Full explanation for S.39
S.53

Your boiler has temporarily paused because it cannot sense enough water flow to operate safely.

Medium DIY-safe £0-180

What to check first

  1. Check the pressure gauge on the front of the boiler
  2. If the pressure is below 1.5 bar, use the filling loop handles underneath the boiler to top it up
  3. Ensure all radiator valves are open to allow water to circulate freely

Parts commonly replaced

  • Circulation Pump
  • Water Pressure Sensor
  • Main Heat Exchanger (Cleaning)

Related codes

Full explanation for S.53
S.54

Your boiler has paused briefly because the water inside is heating up too quickly, which usually means the water isn't flowing through the system as fast as it should.

Medium DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar on the gauge
  3. Check for any strange noises that might suggest air is trapped in the pump or radiators

Parts commonly replaced

  • Circulation pump
  • Central heating filter (cleaning)
  • Flow/Return thermistors

Related codes

Full explanation for S.54