BoilerFaultCodes Communication fault
No communication with the PCB Communication fault between display and PCB in the electronics box
Vaillant ecoTEC plus 637 Fault codes & diagnostics
61 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
System Gas 2012 onwards Discontinued
GC Number 41-044-65
Download the Vaillant ecoTEC plus 637 manual
PDF
Versions & GC numbers
The fault code is already on your display
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
31 High
3 Medium
4 Low
22 Unrated
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 |
|---|---|---|
| 41-044-65 | ecoTEC plus 637 VU GB 376/5-5 | 2012–2015 |
Appliance names and production years are taken from the UK Product Characteristics Database (the SAP/SEDBUK register). Where a GC number is also printed in this model's own manufacturer manual, both sources were checked to agree.
Just want the most common Vaillant ecoTEC plus 637 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 61 documented codes
Tap any card for details
F.00
Fault list
F.01
Return temperature sensor interruption
F.02
Interruption cylinder charging sensor actoSTOR (NTC) only in combination with F.91
F.03
Interruption cylinder sensor actoSTOR (NTC) only in combination with F.91
F.10 Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water leaving the unit, causing it to shut down for safety.
Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water leaving the unit, causing it to shut down for safety.
Parts commonly replaced
- Flow NTC Thermistor
- Wiring harness
- Main PCB
Related codes
F.11 Your boiler has stopped working because a sensor that monitors the temperature of the water returning to the unit has developed an electrical fault.
Your boiler has stopped working because a sensor that monitors the temperature of the water returning to the unit has developed an electrical fault.
Parts commonly replaced
- Return NTC thermistor (sensor)
- Wiring harness
- Main PCB
Related codes
F.12
Short circuit at cylinder charging sensor (NTC) only in combination with F.91
F.13 Your boiler is having trouble reading the temperature of your hot water cylinder because a sensor connection is faulty or shorted.
Your boiler is having trouble reading the temperature of your hot water cylinder because a sensor connection is faulty or shorted.
Parts commonly replaced
- Hot water cylinder sensor
- Wiring harness
Related codes
F.20 Your boiler has shut down because it has detected it is getting too hot, usually caused by a lack of water flow or a blockage in the system.
Your boiler has shut down because it has detected it is getting too hot, usually caused by a lack of water flow or a blockage in the system.
What to check first
- Check if your radiator valves are open
- Check the boiler pressure gauge and top up if it is below 1.0 bar
- Press the reset button (flame symbol) to see if the fault clears
Parts commonly replaced
- Overheat Thermostat
- NTC Temperature Sensor
- Circulation Pump
Related codes
Showing 11–20 of 61
F.22 Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
What to check first
- Locate the filling loop valves underneath your boiler
- Slowly open both valves to let water into the system
- Close both valves tightly once the pressure gauge reaches 1.5 bar
Parts commonly replaced
- Filling loop
- Pressure sensor
- Expansion vessel
Related codes
F.23 Your boiler has switched itself off because the water temperature inside it is rising much faster than the water temperature being sent to your radiators, usually due to a circulation blockage.
Your boiler has switched itself off because the water temperature inside it is rising much faster than the water temperature being sent to your radiators, usually due to a circulation blockage.
What to check first
- Check that all radiator valves (TRVs) are turned to the highest setting
- Ensure the boiler pressure is between 1.0 and 1.5 bar on the digital display
- Check if any valves underneath the boiler have been accidentally closed
Parts commonly replaced
- Pump
- Flow and Return Thermistors
- Main Heat Exchanger (Cleaning/Descaling)
Related codes
F.24 Your boiler has shut down because it is heating up much faster than it should, which usually means the water isn't moving through the system properly.
Your boiler has shut down because it is heating up much faster than it should, which usually means the water isn't moving through the system properly.
What to check first
- Check that your boiler pressure is between 1.0 and 1.5 bar on the digital display
- Ensure all radiator valves are fully open to allow water to flow
- Press the reset button (flame symbol with a cross) to see if the fault clears
Parts commonly replaced
- Circulating Pump
- Temperature Sensors (NTC Thermistors)
- Main Heat Exchanger (Flushing)
Related codes
F.25 Your boiler has detected a loose or broken electrical connection inside its main combustion chamber, meaning it cannot safely turn on the burner.
Your boiler has detected a loose or broken electrical connection inside its main combustion chamber, meaning it cannot safely turn on the burner.
Parts commonly replaced
- Wiring harness
- Combustion sensor
- Gas valve assembly
Related codes
F.26
Fault: Gas valve without function
F.27 Your boiler has sensed a flame inside when there shouldn't be one, so it has shut down as a safety precaution.
Your boiler has sensed a flame inside when there shouldn't be one, so it has shut down as a safety precaution.
Parts commonly replaced
- Ionisation electrode
- Ignition lead
- Gas valve
- Printed Circuit Board (PCB)
Related codes
F.28 Your boiler has failed to ignite and has locked itself out for safety, meaning you currently have no heating or hot water.
Your boiler has failed to ignite and has locked itself out for safety, meaning you currently have no heating or hot water.
What to check first
- Check if your home has gas by testing another gas appliance like a hob.
- Ensure your gas meter has credit and the isolation valve is open.
- Wait 5 minutes and then press the 'Reset' button (indicated by a flame with a cross through it).
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- Spark Lead
- PCB
Related codes
F.29 Your boiler was working but has suddenly turned off because the flame has gone out and it cannot manage to relight itself.
Your boiler was working but has suddenly turned off because the flame has gone out and it cannot manage to relight itself.
What to check first
- Check if other gas appliances, like a hob, are still working to ensure your gas supply hasn't been interrupted.
- If it is freezing outside, check the condensate pipe (the plastic pipe leading outside) for ice and defrost it with warm water.
- Press the reset button (a flame symbol with a cross through it) for five seconds to see if the boiler restarts.
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- Flame Sensing Electrode
- PCB
Related codes
F.32 Your boiler has stopped working because the internal fan that safely removes exhaust gases is either stuck or not communicating correctly with the system.
Your boiler has stopped working because the internal fan that safely removes exhaust gases is either stuck or not communicating correctly with the system.
Parts commonly replaced
- Fan assembly
- Printed Circuit Board (PCB)
- Wiring harness
F.35
Air/flue gas duct fault
Showing 21–30 of 61
F.42
Coding resistor fault (possible in combination with F.70)
F.49 Your boiler has lost communication between its internal computer and parts like the thermostat due to an electrical connection issue.
Your boiler has lost communication between its internal computer and parts like the thermostat due to an electrical connection issue.
Parts commonly replaced
- eBUS wiring
- Digital thermostat
- Main PCB (Printed Circuit Board)
Related codes
F.52
Mass flow sensor connection fault
F.53
Mass flow sensor fault
F.54
Gas pressure fault (in combination with F.28/F.29)
F.56
Fault: Mass flow sensor regulation
F.57
Fault during comfort safety mode
F.61 Your boiler is struggling to talk to the gas valve, which means it cannot safely control the fuel needed to create heat.
Your boiler is struggling to talk to the gas valve, which means it cannot safely control the fuel needed to create heat.
Parts commonly replaced
- Gas Valve
- Main PCB
- Wiring Loom
Related codes
F.62 Your boiler has detected that the gas flame is not turning off when it should, which is preventing the system from running safely.
Your boiler has detected that the gas flame is not turning off when it should, which is preventing the system from running safely.
Parts commonly replaced
- Gas valve
- Printed Circuit Board (PCB)
- Electrode
Related codes
F.63 Your boiler's computer memory has suffered a technical glitch, meaning it can no longer remember its own settings and cannot operate.
Your boiler's computer memory has suffered a technical glitch, meaning it can no longer remember its own settings and cannot operate.
Parts commonly replaced
- Main Printed Circuit Board (PCB)
Related codes
Showing 31–40 of 61
F.64 Your boiler has detected a communication problem between its internal sensors or a fault in its main control board, causing the system to shut down for safety.
Your boiler has detected a communication problem between its internal sensors or a fault in its main control board, causing the system to shut down for safety.
Parts commonly replaced
- Flow or Return NTC Thermistor
- Printed Circuit Board (PCB)
- Wiring Harness
Related codes
F.65 Your boiler's internal control board is getting too hot, usually because it is located too close to a heat source or there is a fault with the electronics.
Your boiler's internal control board is getting too hot, usually because it is located too close to a heat source or there is a fault with the electronics.
Parts commonly replaced
- Printed Circuit Board (PCB)
- Electronic Control Box
Related codes
F.67 Your boiler's internal computer is receiving a confusing signal regarding the flame, preventing the system from starting for safety reasons.
Your boiler's internal computer is receiving a confusing signal regarding the flame, preventing the system from starting for safety reasons.
Parts commonly replaced
- Printed Circuit Board (PCB)
- Ignition Electrode
- Wiring Harness
Related codes
F.68
Unstable flame signal fault Air in gas, gas flow pressure too low, wrong air ratio, condensate route blocked, wrong gas restrictor, ionisation flow interruption (cables, electrodes), flue gas recirculation, condensate route
F.70 Your boiler's computer and internal display screen are no longer communicating correctly because they haven't been configured to recognize each other.
Your boiler's computer and internal display screen are no longer communicating correctly because they haven't been configured to recognize each other.
Parts commonly replaced
- Printed Circuit Board (PCB)
- User Interface (UI) Display
Related codes
F.71 Your boiler has detected that a temperature sensor has stuck on a single reading, meaning it cannot accurately tell how hot the water is to safely heat your home.
Your boiler has detected that a temperature sensor has stuck on a single reading, meaning it cannot accurately tell how hot the water is to safely heat your home.
Parts commonly replaced
- Flow NTC Temperature Sensor
- Sensor Wiring Harness
Related codes
F.72 Your boiler has detected that the temperature readings between two sensors don't match up, which prevents it from heating your water safely.
Your boiler has detected that the temperature readings between two sensors don't match up, which prevents it from heating your water safely.
Parts commonly replaced
- Flow NTC sensor
- Return NTC sensor
- Sensor wiring harness
Related codes
F.73 Your boiler is unable to accurately measure the water pressure because the internal sensor is sending an incorrect or missing signal.
Your boiler is unable to accurately measure the water pressure because the internal sensor is sending an incorrect or missing signal.
Parts commonly replaced
- Water Pressure Sensor
- Sensor Wiring Harness
Related codes
F.74 Your boiler has detected a problem with the sensor that monitors water pressure, meaning it cannot accurately tell if there is enough water in the system to run safely.
Your boiler has detected a problem with the sensor that monitors water pressure, meaning it cannot accurately tell if there is enough water in the system to run safely.
Parts commonly replaced
- Water pressure sensor
- Wiring harness
Related codes
F.75 Your boiler is unable to tell if its pump is running correctly, which means it has stopped working for your safety.
Your boiler is unable to tell if its pump is running correctly, which means it has stopped working for your safety.
What to check first
- Check the pressure gauge and ensure it is between 1.0 and 1.5 bar
- If the pressure is too low, use the filling loop to top it up
- Press the 'Reset' button (flame symbol with a cross) once the pressure is correct
Parts commonly replaced
- Water pressure sensor
- Circulation pump
- Expansion vessel (recharge or replace)
Related codes
Showing 41–50 of 61
F.76 Your boiler has shut down because a safety sensor has detected that the main heat exchanger is getting too hot.
Your boiler has shut down because a safety sensor has detected that the main heat exchanger is getting too hot.
Parts commonly replaced
- Overheat Safety Thermal Fuse
- Primary Heat Exchanger
- Main PCB
Related codes
F.77 Your boiler has detected an issue with the drainage system that removes leftover liquid from the exhaust pipe or a fault with the safety flap inside the flue.
Your boiler has detected an issue with the drainage system that removes leftover liquid from the exhaust pipe or a fault with the safety flap inside the flue.
Parts commonly replaced
- Condensate pump
- Flue non-return flap
- Wiring harness
Related codes
F.78 Your boiler is struggling to communicate with an external control box used for managing your hot water or heating zones, preventing it from functioning correctly.
Your boiler is struggling to communicate with an external control box used for managing your hot water or heating zones, preventing it from functioning correctly.
Parts commonly replaced
- VR65 Wiring Centre
- eBUS cable
- Main PCB
Related codes
F.83
Flow and/or return temperature sensor temperature change fault When the burner starts, the temperature change registered at flow and/or return temperature sensor is non-existent or too small. - Not enough water in the boiler - Flow and/return temperature sensor not in correct position at pipe.
F.84
Fault: Flow/return temperature sensor temperature difference implausible Flow and return temperature sensors returning implausible values. - Flow and return temperature sensors have been inverted. - Flow and return temperature sensors have not been correctly fitted.
F.85
Fault: Flow and return temperature sensors incorrectly fitted Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe.
F.91
Cylinder sensor/charging sensor related fault (referenced in F.02, F.03, F.12, F.13)
F.92
Coding resistor fault: The coding resistor on the PCB does not match the entered gas family
F.93
Gas group fault: Combustion quality outside the permitted range
S.04
Heating mode: Burner on
Showing 51–60 of 61
S.14 Your boiler is operating normally and has successfully lit the flame to heat your water or radiators.
Your boiler is operating normally and has successfully lit the flame to heat your water or radiators.
Related codes
S.20 Your boiler is simply letting you know that the 'warm start' feature is active, which keeps a small amount of water pre-heated so you get hot water at your taps more quickly.
Your boiler is simply letting you know that the 'warm start' feature is active, which keeps a small amount of water pre-heated so you get hot water at your taps more quickly.
Related codes
S.21 Your boiler is struggling to start the fan when you turn on your hot water taps, meaning the system cannot safely clear exhaust gases to begin heating.
Your boiler is struggling to start the fan when you turn on your hot water taps, meaning the system cannot safely clear exhaust gases to begin heating.
Parts commonly replaced
- Fan Assembly
- Printed Circuit Board (PCB)
- Wiring Harness
Related codes
S.22 Your boiler has detected that hot water is being called for, but it believes the pump is running when it shouldn't be or there is a circulation issue preventing proper operation.
Your boiler has detected that hot water is being called for, but it believes the pump is running when it shouldn't be or there is a circulation issue preventing proper operation.
Parts commonly replaced
- Internal Pump
- Main PCB
- Flow Sensor
Related codes
S.23 Your boiler is having trouble lighting the flame when you turn on your hot water taps.
Your boiler is having trouble lighting the flame when you turn on your hot water taps.
Parts commonly replaced
- Ignition electrode
- Flame sensor
- Gas valve
- Spark lead
Related codes
S.24 Your boiler is struggling to keep the flame lit while trying to heat your tap water, which usually means the system is cutting out during use.
Your boiler is struggling to keep the flame lit while trying to heat your tap water, which usually means the system is cutting out during use.
Parts commonly replaced
- Gas valve
- Ignition lead
- Electrode
- Burner gasket
Related codes
S.25 Your boiler is temporarily cooling itself down after finishing a cycle for hot water.
Your boiler is temporarily cooling itself down after finishing a cycle for hot water.
What to check first
- Check that at least one radiator or bypass valve is open to allow heat to dissipate
- Wait for the code to disappear naturally as the boiler settles
- Ensure there is enough water pressure shown on the digital display (1.0 to 1.5 bar)
Parts commonly replaced
- None (Normal operation)
- Pump (if code persists)
- Diverter Valve
Related codes
S.34 Your boiler has paused operation because it believes the internal temperature is so low that the pipes are at risk of freezing.
Your boiler has paused operation because it believes the internal temperature is so low that the pipes are at risk of freezing.
What to check first
- Check that the room thermostat and radiator valves are turned up
- Ensure the boiler's power supply is constant and has not been switched off
- If the condensate pipe is outside, check if it is frozen and gently thaw it with warm water
- Wait 15-30 minutes for the boiler to warm up naturally before resetting
Parts commonly replaced
- NTC Temperature Sensor
- Condensate Trace Heater
- PCB
Related codes
S.42 Your boiler has temporarily paused because a safety flap in the flue system is not providing the correct signal to allow the burner to start.
Your boiler has temporarily paused because a safety flap in the flue system is not providing the correct signal to allow the burner to start.
Parts commonly replaced
- Flue non-return flap
- Flue gas sensor
- Wiring harness
Related codes
S.53 Your boiler is pausing temporarily because it is struggling to circulate water quickly enough to keep the system cool.
Your boiler is pausing temporarily because it is struggling to circulate water quickly enough to keep the system cool.
What to check first
- Check that the boiler pressure is between 1.0 and 1.5 bar on the display
- Ensure all radiator valves (TRVs) are fully open to allow water flow
- Check that the blue isolation valves underneath the boiler are in the 'open' position
Parts commonly replaced
- Central heating pump
- Main heat exchanger (flushing/cleaning)
- Pressure sensor
Related codes
Showing 61–61 of 61
S.54 Your boiler has temporarily paused operation because it detected a sudden rise in temperature and is waiting for the system to cool down or balance itself.
Your boiler has temporarily paused operation because it detected a sudden rise in temperature and is waiting for the system to cool down or balance itself.
What to check first
- Ensure all radiator valves (TRVs) are fully open to allow water to flow.
- Check that the boiler pressure is between 1.0 and 1.5 bar on the digital display.
- Check for any trapped air by bleeding your radiators.
Parts commonly replaced
- External bypass valve
- Central heating pump
- Temperature sensors (NTC thermistors)
Related codes
Status & diagnostic codes 16 operating states & engineer diagnostics
Not faults — these are normal operating states. Tap to expand.
16 operating states & engineer diagnostics
Not faults — these are normal operating states. Tap to expand.
d.71 Diagnostic Your boiler is simply indicating that the heating temperature dial is currently turned to its maximum setting. S.10 Status Your boiler has detected that you have turned on a hot water tap and is currently working to provide hot water to your home. S.11 Status Your boiler is trying to heat your water, but the internal fan responsible for safety cannot get started. S.15 Status Your boiler is briefly keeping the internal pump and fan running to clear away excess heat after you have finished using hot water. S.16 Status Your boiler's fan is running for a short period after the burner has turned off to clear any leftover gases safely. S.17 Status Your boiler is currently cooling itself down by running the water pump after a heating cycle, which is a normal part of the system's operation. S.26 Status Your boiler is currently running its internal fan to cool down the system after finishing a cycle, which is a normal part of the shutdown process. S.27 Status Your boiler is currently cooling itself down by circulating water, which is a normal process and not an error. S.28 Status Your boiler is taking a short, programmed break to prevent the burner from starting and stopping too frequently while heating your water. S.30 Status Your boiler is working correctly but is currently in a 'waiting' state because your room thermostat is telling it that the house is already warm enough. S.31 Status Your boiler is currently set to only provide hot water and is ignoring your request for central heating. S.32 Status Your boiler is temporarily pausing because the internal fan is not spinning at the correct speed needed to safely light the burner. S.41 Status Your boiler has too much water in the system, causing the pressure to rise to a level that is higher than recommended for safe operation. S.96 Status Your boiler is currently running a routine internal self-check on its water temperature sensors and has temporarily paused your heating or hot water to complete this process. S.97 Status Your boiler is currently performing a routine internal safety check on its water pressure system, so it is temporarily pausing your heating and hot water. S.98 Status Your boiler is currently performing an internal health check to make sure its temperature sensors are reading correctly before it restarts.