Ideal Logic Max System Fault codes & diagnostics

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

System Natural Gas 15-30 kW 2018-present
GC Number 41-796-13
Download the Ideal Logic Max System manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 50 High 2 Medium 3 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
41-796-13 LOGIC MAX SYSTEM S15 2018–present
Just want the most common Ideal Logic Max System faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 56 documented codes

Tap any card for details

4.2

Your boiler has shut down because its internal temperature has become too high, often caused by a lack of water flow or a blockage.

High DIY-safe £100-350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the gauge
  2. Ensure all radiator valves and the isolation valves under the boiler are fully open
  3. Press the 'Restart' button on the front control panel

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger

Related codes

Full explanation for 4.2
4.3

Your boiler has failed to light after several attempts, meaning it has shut down as a safety precaution and will not provide heating or hot water.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, such as a gas hob, are working to ensure gas is reaching the property.
  2. Ensure your pre-payment gas meter has credit if applicable.
  3. Press the 'Reset' button on the boiler control panel once to see if it restarts. Only attempt this three times maximum.
  4. If it is freezing outside, check that your white plastic condensate pipe leading outdoors is not frozen and blocked.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing probe
  • PCB (Printed Circuit Board)

Related codes

Full explanation for 4.3
4.4

Your boiler has detected a flame when it shouldn't be there, which usually means a internal component is stuck open or there is a fault with the electronics.

Emergency Engineer only £180-350

Parts commonly replaced

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

Related codes

Full explanation for 4.4
4.5

Your boiler has detected that the water pressure inside the system is too low to run safely and has switched itself off.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually blue-handled levers) underneath the boiler.
  2. Slowly open the valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves when the needle reaches 1.5 bar.
  4. Restart the boiler if it does not reset automatically.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 4.5
4.6

Your boiler is struggling to keep its flame lit, which means the system has safely shut down to prevent issues.

High DIY-safe £120-£350

What to check first

  1. Check that your gas credit hasn't run out if you have a prepayment meter
  2. Ensure other gas appliances like your hob are working to confirm your gas supply is active
  3. Check that the white plastic condensate pipe outside hasn't frozen in cold weather
  4. Press the 'Reset' button on the boiler display

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • Spark generator

Related codes

Full explanation for 4.6
4.7

Your boiler has stopped working because the internal fan, which safely clears exhaust fumes, is not running correctly.

High Engineer only £280-450

Parts commonly replaced

  • Fan Assembly
  • Main PCB
  • Wiring Loom

Related codes

Full explanation for 4.7
4.8

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, preventing it from heating your home safely.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness / connector
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 4.8
4.9

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, likely causing it to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for 4.9
4.10

Your boiler has lost communication with the sensor that monitors the outdoor temperature, which means it cannot automatically adjust its settings based on the weather.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for 4.10
4.11

Your boiler has detected that the water pressure in the system is too low and needs to be topped up to work properly.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath your boiler.
  2. Slowly open the blue-handled valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches 1.5 bar.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 4.11
Showing 11–20 of 56
4.12

Your boiler has stopped working because it cannot detect water moving through the internal pipes to keep the system safe.

High DIY-safe £100-£350

What to check first

  1. Check the pressure gauge on the front of the boiler is between 1 and 1.5 bar
  2. If the pressure is low, use the filling loop underneath the boiler to top it up
  3. Ensure all radiator valves and isolation valves under the boiler are fully open
  4. Press the Restart button to clear the fault

Parts commonly replaced

  • Pump
  • Water Pressure Sensor
  • Automatic Air Vent

Related codes

Full explanation for 4.12
4.13

Your boiler's internal computer parts are failing to talk to each other, meaning the system cannot operate or provide heat and hot water.

High Engineer only £180-450

Parts commonly replaced

  • Primary PCB
  • Wiring Loom
  • User Interface Board

Related codes

Full explanation for 4.13
4.14

Your boiler is having trouble measuring the temperature of your hot water, which usually means it cannot heat water safely or effectively.

High Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring harness

Related codes

Full explanation for 4.14
4.15

Your boiler is heating your radiators normally, but it is failing to provide hot water when you turn on your taps.

Medium Engineer only £120-£250

Parts commonly replaced

  • Diverter Valve Actuator Motor
  • Diverter Valve Cartridge
  • DHW Flow Turbine

Related codes

Full explanation for 4.15
4.16

Your boiler has lost all power or the control screen has failed, meaning the system will not turn on or provide heat.

High DIY-safe £100-350

What to check first

  1. Check if the main boiler power switch or fused spur next to the boiler is turned on
  2. Check your home's consumer unit (fuse box) to see if a circuit breaker has tripped
  3. Replace the 3-amp fuse in the boiler's power spur switch

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display User Interface
  • Internal Fuse

Related codes

Full explanation for 4.16
Blocked Flue/Condensate

Your boiler is unable to get rid of waste gases or waste water safely, which has caused it to shut down for protection.

High DIY-safe £80-180

What to check first

  1. Check the white plastic pipe outside for ice and pour warm water over it if frozen
  2. Ensure the external flue terminal is not obstructed by leaves, nests, or debris
  3. Check that the internal condensate trap isn't visibly full of sludge

Parts commonly replaced

  • Condensate Trap
  • Flue Thermistor
  • Pressure Switch

Related codes

Full explanation for Blocked Flue/Condensate
F 0A

Your boiler has detected that the water is flowing in the wrong direction through the pipes or that the temperature sensors have been installed on the wrong pipes.

High Engineer only £100-250

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Pump

Related codes

Full explanation for F 0A
F 0t

Your boiler is having trouble communicating with your wall thermostat, meaning it doesn't know when to turn your heating on or off.

Medium DIY-safe £60-220

What to check first

  1. Check if your room thermostat is switched on and hasn't run out of batteries.
  2. Ensure the thermostat is set to a temperature higher than the current room temperature.
  3. If using a wireless thermostat, check that the receiver next to the boiler is showing power and try re-pairing the device.

Parts commonly replaced

  • Batteries
  • Room Thermostat
  • Wired connections/cabling

Related codes

Full explanation for F 0t
F 0U

Your boiler is overheating because the water is not flowing through the system quickly enough to be cooled down.

High DIY-safe £120-350

What to check first

  1. Ensure all radiator valves (TRVs) are turned to the highest setting
  2. Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge
  3. Switch the boiler off and back on at the fused spur to reset the system

Parts commonly replaced

  • Central Heating Pump
  • Main Heat Exchanger (Descaling)
  • Flow and Return Thermistor

Related codes

Full explanation for F 0U
F 04 OR L 04

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely control the heat.

High Engineer only £90-160

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for F 04 OR L 04
Showing 21–30 of 56
F 05 OR L 05

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, causing it to stop working for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness
  • Control box (PCB)

Related codes

Full explanation for F 05 OR L 05
F0 d

Your boiler has detected that water is not moving through the system properly, which has caused it to stop working for safety.

High DIY-safe £100-£350

What to check first

  1. Check that your boiler pressure gauge is between 1 and 1.5 bar
  2. Ensure all radiator valves are fully open
  3. Verify that the main stopcock and internal isolation valves are open
  4. Turn the power off and back on to reset the system

Parts commonly replaced

  • Circulating Pump
  • Flow Thermistor
  • Water Pressure Sensor

Related codes

Full explanation for F0 d
F01

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-80

What to check first

  1. Locate the external filling loop, which is a silver braided hose connected to your boiler pipes.
  2. Open the small tap or valve on the filling loop to allow water to enter the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).
  4. Close the tap firmly to stop the flow of water.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for F01
F02

Your boiler has started an ignition process successfully but then lost the flame, meaning it cannot stay lit to provide heat or hot water.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances, like a cooker hob, are working to ensure gas is being supplied to the property.
  2. Verify that your gas meter has credit if you use a pre-payment meter.
  3. Reset the boiler by turning the control knob to the 'RESET' position and holding for five seconds.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for F02
F03

Your boiler has stopped working because the fan that removes waste gases is not spinning correctly, causing the system to shut down for safety.

High Engineer only £220-380

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Venturi tube

Related codes

Full explanation for F03
F04

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, which means it cannot heat your home or water safely.

High Engineer only £90-160

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for F04
F04 L04

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, usually causing the system to shut down as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Thermistor wiring harness

Related codes

Full explanation for F04 L04
F05

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, causing it to stop working for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness / connector

Related codes

Full explanation for F05
F05 L05

Your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing it from heating up correctly.

High Engineer only £90-160

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring harness connector

Related codes

Full explanation for F05 L05
F06

Your boiler's outdoor temperature monitor is not sending a signal, which may cause your heating to behave unpredictably based on the weather.

Low Engineer only £100-180

Parts commonly replaced

  • Outside sensor (Thermistor)
  • Wiring harness

Related codes

Full explanation for F06
Showing 31–40 of 56
F07

Your boiler has detected that the electrical supply from the grid is currently too low for it to operate safely.

High Engineer only £0-120

Parts commonly replaced

  • External main fuse box
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F07
F09

Your boiler's internal computer has developed a fault and can no longer control the heating or hot water safely.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for F09
F09 L09

Your boiler's main control board has lost its programmed settings and no longer knows how to operate the specific model it is installed in.

High Engineer only £120-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for F09 L09
F10

Your boiler is unable to create the electric spark needed to light the gas burner, meaning your heating and hot water will not work.

High Engineer only £130-£220

Parts commonly replaced

  • Spark Generator
  • Ignition Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F10
Fan Fault

Your boiler has detected a problem with the fan that clears exhaust gases, preventing the system from starting up for safety reasons.

High Engineer only £220-450

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

Full explanation for Fan Fault
Flame On Before Gas Valve On

Your boiler's safety system has detected a flame signal when there shouldn't be one, which prevents the unit from starting up for safety reasons.

High Engineer only £120-£280

Parts commonly replaced

  • Ionisation electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Flame On Before Gas Valve On
Flow Thermistor Fault

Your boiler has detected that a temperature sensor is not communicating correctly, which has caused the system to stop working to prevent overheating.

High Engineer only £90-180

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for Flow Thermistor Fault
Fn

Your boiler has lost its flame while trying to run, which means it cannot heat your water or your radiators.

High DIY-safe £120-350

What to check first

  1. Check that your gas supply is turned on at the meter
  2. If you have a prepay meter, check that you have enough credit
  3. Press the Reset button on the boiler display
  4. Ensure your external condensate pipe hasn't frozen in very cold weather

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Flame Sensing Probe
  • Spark Lead

Related codes

Full explanation for Fn
L 0C

Your boiler is repeatedly shutting down and trying to restart itself, likely due to an underlying component failure preventing reliable ignition.

High DIY-safe £120-£350

What to check first

  1. Turn the power to the boiler off at the fused spur switch (the wall switch).
  2. Wait for at least 5 minutes to allow the internal timer to reset.
  3. Turn the power back on and wait for the boiler to complete its start-up sequence without pressing the reset button.

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • PCB

Related codes

Full explanation for L 0C
L 0d

Your boiler has stopped working because it cannot safely get rid of exhaust fumes or waste water, often due to a blockage or a frozen pipe.

High DIY-safe £100-280

What to check first

  1. Check the white plastic condensate pipe outside for ice and thaw it with warm (not boiling) water.
  2. Check that the external flue terminal is not obstructed by debris or snow.
  3. Reset the boiler by turning the control knob to 'RE' or pressing the reset button.

Parts commonly replaced

  • Condensate Trap
  • Ignition Electrode
  • Gas Valve
  • PCB

Related codes

Full explanation for L 0d
Showing 41–50 of 56
L0

Your boiler has stopped working because either its exhaust pipe is obstructed or the pipe that drains away waste water is blocked.

High DIY-safe £80-220

What to check first

  1. Check the white plastic pipe outside for ice or debris and gently clear it
  2. Thaw any frozen external pipes using a hot water bottle or warm water
  3. Press the 'Restart' button on the boiler display
  4. Ensure the external flue terminal is not covered by leaves or nests

Parts commonly replaced

  • Condensate trap
  • Flue fan
  • Air Pressure Switch

Related codes

Full explanation for L0
L1

Your boiler has switched itself off because the internal water temperature has become too hot, usually caused by a lack of water flow through the system.

High DIY-safe £120-£350

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure your boiler pressure is between 1 and 1.5 bar on the gauge
  3. Reset the boiler by turning the control knob to 'RE' or pressing the reset button

Parts commonly replaced

  • Circulating pump
  • Thermistor (NTC sensor)
  • Main heat exchanger

Related codes

Full explanation for L1
L01

Your boiler has switched itself off because internal sensors have detected it is getting too hot, which is a safety measure to prevent damage.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure there are no blockages or trapped air by bleeding your radiators
  3. Once the boiler has cooled down, press the 'Restart' button on the front panel

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger

Related codes

Full explanation for L01
L2

Your boiler has tried to light several times but cannot establish a flame, meaning it has shut down for safety to prevent gas build-up.

High DIY-safe £100-280

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. If you have a prepay meter, ensure you have sufficient credit.
  3. In freezing temperatures, check if the external condensate pipe has frozen.
  4. Reset the boiler by turning the mode knob to 'RE' and back to your desired setting.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Ignition Lead
  • Ionisation Probe

Related codes

Full explanation for L2
L02

Your boiler has failed to light several times and has locked itself for safety, meaning you have no heating or hot water.

High DIY-safe £100-350

What to check first

  1. Check that your gas supply is turned on (check other gas appliances like a hob).
  2. If you have a prepay meter, ensure there is sufficient credit.
  3. Press and hold the 'Reset' button for 2 seconds to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Generator
  • Detection Lead

Related codes

Full explanation for L02
L04

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water leaving the unit, causing it to stop working for safety.

High Engineer only £90-160

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for L04
L05

Your boiler has lost its ability to accurately measure the temperature of the water coming back into the system, which is preventing it from heating up correctly.

High Engineer only £90-£160

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for L05
L06

Your boiler is detecting a flame even though the gas valve should be closed, which is a safety precaution that stops the boiler from running.

High Engineer only £150-£350

Parts commonly replaced

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

Related codes

Full explanation for L06
L09

Your boiler's internal computer brain is not communicating correctly with the rest of the unit and needs to be officially set up.

High Engineer only £100-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for L09
Ln

Your boiler has lost its flame while running, meaning it is not getting enough gas or the spark is failing to keep the burner lit.

High DIY-safe £100-350

What to check first

  1. Check if your gas meter has credit (if on a prepayment meter)
  2. Check if other gas appliances, like a hob, are working to confirm gas supply
  3. Reset the boiler by turning the mode knob to 'RE' and then back to the desired setting
  4. Ensure the external condensate pipe has not frozen if it is winter

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

Full explanation for Ln
Showing 51–56 of 56
LO2

Your boiler is likely unable to drain away the condensation it produces naturally, causing it to 'choke' and shut down.

High DIY-safe £80-160

What to check first

  1. Locate the plastic pipe leading from the boiler to your outside drain.
  2. If it is freezing cold outside, gently pour warm (not boiling) water over the outdoor section of this pipe to melt any ice blockages.
  3. Restart the boiler by pressing the reset button.

Parts commonly replaced

  • Condensate trap
  • Condensate pipework
  • Spark electrode

Related codes

Full explanation for LO2
No Water Flow

Your boiler has stopped working because it cannot detect water moving through the pipes properly.

High DIY-safe £120-280

What to check first

  1. Check that the central heating pressure gauge is between 1 and 1.5 bar
  2. Ensure all radiator valves and boiler isolation valves are fully open
  3. Gently bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Circulating Pump
  • Water Flow Sensor
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for No Water Flow
Outside Sensor Fault

Your boiler is struggling to understand the outdoor temperature, which might make your heating feel less efficient or slightly inconsistent.

Low Engineer only £90-180

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for Outside Sensor Fault
PCB Fault

Your boiler's main internal computer has developed a serious glitch or hardware failure, meaning the unit cannot safely manage its functions.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for PCB Fault
Return Thermistor Fault

Your boiler is struggling to monitor the temperature of the water returning from your radiators, which prevents it from heating your home or water efficiently.

High Engineer only £110-180

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for Return Thermistor Fault
Room Thermostat Fault

Your boiler has lost communication with your heating controls, meaning your radiators might not turn on when you want them to.

High DIY-safe £5-80

What to check first

  1. Locate your wall-mounted room thermostat or portable controller
  2. Open the battery compartment (usually by sliding a cover or pressing a small tab)
  3. Replace the old batteries with a brand new set of high-quality alkaline batteries
  4. Check if the display on the thermostat turns back on and try to call for heat

Parts commonly replaced

  • AA or AAA batteries
  • Wireless room thermostat

Related codes

Full explanation for Room Thermostat Fault
Status & diagnostic codes

15 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

2S01 Diagnostic Your boiler is notifying you that it has been configured to be controlled by a single external timer or thermostat wire rather than a smart system. 2S02 Diagnostic Your boiler is confused by how your heating controls are wired and thinks it should be receiving two different signals to turn on at once. dU Diagnostic Your boiler is currently set to a specific maintenance mode used by engineers to replace internal parts, rather than being in a fault state. F9 Diagnostic Your boiler’s internal control board has lost its configuration or is brand new and hasn't been programmed yet to tell it which model it is. FL Status Your boiler is accidentally trying to use a feature it doesn't have, which is usually caused by a minor electronic glitch or a recent settings change. FP Status Your boiler has detected that the temperature is very low and has automatically turned itself on to prevent the internal pipes from freezing. oF Status Your boiler is currently updating its internal settings and is showing this code to indicate it has been reset during its configuration process. P1 Diagnostic Your boiler is displaying a service setting notification to run the pump at maximum speed, which usually indicates an installation or maintenance mode rather than a dangerous mechanical failure. P7 Diagnostic Your boiler is displaying a configuration message regarding how fast its internal pump should run to move water through your radiators. PH Diagnostic Your boiler is not broken; it is simply in 'Installer Mode' which is used during setup or testing and can be exited by adjusting the control knobs. SE Diagnostic Your boiler is simply reminding you that its annual service is due soon; it is a notification rather than a breakdown. SH Diagnostic Your boiler is currently in a manual testing mode where it runs at full power, usually activated by a technician during a service. SL Diagnostic Your boiler is currently in a manual service mode used for testing and is not experiencing an actual fault. UF Diagnostic Your boiler is currently stuck in a professional service mode used by engineers when they are replacing an internal valve. uP Diagnostic Your boiler has detected that it was turned on while its internal computer chip was being replaced or updated.
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