Vokera Syntesi 25 Fault codes & diagnostics

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

Combi Natural Gas 25 kW 2010-2015 Discontinued

Replaced by: Vokera Vision

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Severity at a glance

Each cell = one fault code. Hover to identify.
30 High 3 Medium 1 Low
Just want the most common Vokera Syntesi 25 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 34 documented codes

Tap any card for details

01

Your boiler is unable to light the flame needed to heat your water and radiators, often due to a temporary interruption in the gas supply.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a cooker, are working to confirm gas is reaching the property.
  2. Ensure your gas meter has credit if you use a pre-payment meter.
  3. Press the reset button on the boiler control panel and hold for a few seconds to try and restart the ignition sequence.

Parts commonly replaced

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

Related codes

Full explanation for 01
02

Your boiler has reached an unsafe temperature because the water is not circulating correctly through the system.

High DIY-safe £150-£350

What to check first

  1. Check that the isolation valves under the boiler are in the open position
  2. Ensure all radiator valves (TRVs) are turned up to full
  3. Try resetting the boiler using the control knob to see if the pump clears the blockage

Parts commonly replaced

  • Circulation Pump
  • Limit Thermostat
  • Main Heat Exchanger (if scaled)

Related codes

Full explanation for 02
03

Your boiler has stopped working because the pipe that lets exhaust gases out is blocked or the internal sensor that checks for airflow is clogged with dirt.

High Engineer only £90-180

Parts commonly replaced

  • Fan
  • Air Pressure Switch
  • Venturi Tube
  • Flue Seals

Related codes

Full explanation for 03
04

Your boiler has detected that the water pressure is too low to operate safely, which is usually caused by a leak or air trapped in the system.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (a flexible silver hose) underneath the boiler.
  2. Slowly open the one or two taps on the hose until you hear water flowing.
  3. Watch the pressure gauge and close the taps once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Automatic air vent

Related codes

Full explanation for 04
05

Your boiler is having trouble 'talking' to your wall-mounted thermostat or remote control unit.

High Engineer only £90-180

Parts commonly replaced

  • Remote control unit (RC05)
  • Connecting wiring
  • Main PCB

Related codes

Full explanation for 05
06

Your boiler is having trouble measuring the temperature of your hot water, which is likely causing your taps and showers to stay cold.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring harness connecting to the sensor

Related codes

Full explanation for 06
07

Your boiler's internal temperature sensor is failing to provide a reading, which means the system has shut down to prevent overheating.

High Engineer only £90-160

Parts commonly replaced

  • Primary NTC thermistor
  • Wiring harness

Related codes

Full explanation for 07
08

Your boiler has shut down because a temperature sensor has detected that the burner area is getting too hot.

High Engineer only £120-250

Parts commonly replaced

  • Burner thermostat
  • Main heat exchanger
  • Temperature sensor (NTC)

Related codes

Full explanation for 08
09

Your boiler's electronic control board has developed a fault, meaning the internal computer can no longer communicate with the rest of the system.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for 09
AL10

Your boiler has tried to start up but cannot light the gas to produce heat or hot water.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances like a hob are working to ensure gas is reaching the property
  2. Check if your gas meter has credit or if the emergency control valve is open
  3. Press the 'Reset' button on the boiler control panel to attempt a restart

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for AL10
Showing 11–20 of 34
AL20

Your boiler has shut down because its internal temperature has reached a dangerously high level, triggering a safety switch to prevent damage.

High Engineer only £90-180

Parts commonly replaced

  • Limit thermostat
  • Overheat sensor
  • Circulation pump
  • Main heat exchanger

Related codes

Full explanation for AL20
AL21

Your boiler has shut down because a temperature sensor on the burner has detected an issue or has failed.

High Engineer only £120-£240

Parts commonly replaced

  • Burner thermostat sensor
  • Overheat limit thermostat
  • Main PCB

Related codes

Full explanation for AL21
AL26

Your boiler has stopped working because it thinks the waste water pipe is full or there is an issue with the sensor that detects blockages.

High DIY-safe £90-180

What to check first

  1. Check the external plastic condensate pipe for signs of freezing if the weather is very cold.
  2. Thaw a frozen external pipe using a hot water bottle or warm (not boiling) water.
  3. Check that the condensate discharge pipe is not blocked by debris or leaves.

Parts commonly replaced

  • Condensate level sensor
  • Condensate trap
  • PCB

Related codes

Full explanation for AL26
AL28

Your boiler has lost its ability to monitor the temperature of the water returning to the system, causing it to stop working for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Sensor wiring harness
  • Control board

Related codes

Full explanation for AL28
AL34

Your boiler has stopped working because the fan that removes waste gases is not spinning at the correct speed or has failed to start.

High Engineer only £220-£450

Parts commonly replaced

  • Fan motor
  • PCB (Main Circuit Board)
  • Wiring harness
Full explanation for AL34
AL40

Your boiler has detected that the water pressure is too low to operate safely and has shut down to prevent damage.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Slowly open the one or two taps on the hose until you hear water flowing.
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for AL40
AL41

Your boiler has detected that the water pressure is too low to operate safely and has shut down to prevent damage.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a flexible silver hose) underneath or near the boiler.
  2. Slowly open the one or two taps/valves on the filling loop while watching the pressure gauge.
  3. Close the valves firmly once the pressure gauge needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • External filling loop
  • Pressure sensor
  • Expansion vessel recharge

Related codes

Full explanation for AL41
AL52

Your boiler's internal computer has encountered an electronic failure and cannot safely control the unit.

High Engineer only £250-450

Parts commonly replaced

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

Related codes

Full explanation for AL52
AL55

Your boiler’s internal computer has encountered a critical error and can no longer control the heating system safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Internal Wiring Harness

Related codes

Full explanation for AL55
AL60

Your boiler is having trouble measuring the temperature of your hot water, which usually means your taps will only run cold.

Medium Engineer only £90-160

Parts commonly replaced

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

Related codes

Full explanation for AL60
Showing 21–30 of 34
AL71

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

High Engineer only £90-160

Parts commonly replaced

  • Flow NTC Temperature Sensor
  • Wiring harness / Thermistor

Related codes

Full explanation for AL71
AL73

Your boiler has stopped working because it cannot correctly measure the temperature of the water coming back from your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Return NTC thermistor sensor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for AL73
AL79

Your boiler has likely overheated or a safety sensor is failing, causing the system to lock out to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Flow temperature NTC sensor
  • High limit thermostat
  • Main heat exchanger

Related codes

Full explanation for AL79
CO

Your boiler is currently running in a special engineer-only testing mode for checking gas emissions and is not experiencing a mechanical fault.

Low DIY-safe

What to check first

  1. Turn the selector dial to the 'OFF/RESET' position.
  2. Wait approximately 10 seconds.
  3. Turn the selector dial back to your desired setting (Heating/Hot Water).
Full explanation for CO
d1 & d2

Your boiler has detected a blockage or a backup of water in the condensation drainage pipe, usually caused by a frozen pipe or a clogged trap.

High DIY-safe £80-140

What to check first

  1. Check the white plastic pipe leading outside for signs of freezing or ice blockages.
  2. Thaw any frozen external pipes using a hot water bottle or warm (not boiling) water.
  3. Restart the boiler once the pipe is clear to see if the drainage resumes.

Parts commonly replaced

  • Condensate trap
  • Condensate level sensor
  • Flexible drainage tubing

Related codes

Full explanation for d1 & d2
E01

Your boiler is failing to light the flame needed to produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for E01
E02

Your boiler has detected that the water temperature inside it has become dangerously high and has automatically turned itself off for safety.

High Engineer only £120-250

Parts commonly replaced

  • High limit thermostat
  • Pump
  • Diverter valve
  • Heat exchanger

Related codes

Full explanation for E02
E03

Your boiler is struggling to clear exhaust gases safely, so it has shut down to prevent the build-up of fumes inside your home.

High Engineer only £150-300

Parts commonly replaced

  • Air pressure switch
  • Flue fan assembly
  • Venturi tube
Full explanation for E03
E04

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop, usually two small taps or a braided metal hose under the boiler.
  2. Slowly open the valves to allow mains water into the system until the boiler's pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valves firmly and check that the error code disappears.

Parts commonly replaced

  • Filling loop
  • Pressure gauge
  • Pressure relief valve
Full explanation for E04
E05

Your boiler has stopped working because it can no longer accurately measure the temperature of the water being heated for your radiators.

High Engineer only £120-220

Parts commonly replaced

  • Central heating NTC thermistor

Related codes

Full explanation for E05
Showing 31–34 of 34
E06

Your boiler is having trouble sensing the temperature of the water it is trying to heat for your taps and shower.

Medium Engineer only £120-180

Parts commonly replaced

  • DHW NTC Thermistor

Related codes

Full explanation for E06
E07

Your boiler has lost the ability to accurately measure the temperature of the water flowing through its internal system, so it has safely shut down to prevent overheating.

High Engineer only £120-200

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for E07
E08

Your boiler has lost the ability to accurately measure the temperature of the water returning to it, so it has shut down to prevent potential overheating.

High Engineer only £120-200

Parts commonly replaced

  • Return thermistor sensor
  • Wiring harness

Related codes

Full explanation for E08
E09

Your boiler has detected an electrical issue with the internal component that controls the gas supply, meaning it cannot safely ignite to provide heating or hot water.

High Engineer only £150-250

Parts commonly replaced

  • Gas valve
  • PCB

Related codes

Full explanation for E09
Status & diagnostic codes

2 operating states & engineer diagnostics

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

d1 Diagnostic Your boiler's waste water pipe is blocked or frozen, preventing the system from draining properly and causing it to shut down for safety. d2 Diagnostic Your boiler has detected a blockage or a fault in the system that drains away excess condensation, which has caused it to stop running for safety.