Vaillant Furnaces Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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F.00
Boiler/furnace will not start, the display shows F.00, and no ignition attempt is heard.
Flow Temperature Sensor Open Circuit
F.01
Boiler/furnace locks out with F.01, runs briefly, then shuts down due to overheating.
Flow Temperature Sensor Short Circuit
F.10
Furnace displays F.10 and will not fire, sometimes after overheating or poor circulation.
Return Temperature Sensor Open Circuit
F.11
Furnace displays F.11, may overheat quickly or cycle rapidly.
Return Temperature Sensor Short Circuit
F.20
Boiler shuts down with F.20; casing and flow pipe are very hot, and the pump may be noisy.
Safety Temperature Limiter (STB) / Overheat Protection Activated
F.22
Display shows F.22, boiler will not fire, and the pressure gauge reads very low or zero.
Low Water Pressure / Dry Fire Protection
F.23
F.23 is displayed, the boiler cycles off quickly, and the flow temperature rises much faster than the return.
Flow/Return Temperature Difference Too High (Poor Circulation)
F.24
F.24 error: the boiler overheats rapidly after starting, then shuts down.
Low Water Content / Pump Fault / Overheat
F.25
Boiler locks out with F.25; the fan may run, but the burner does not stay lit.
Flue Gas Temperature Too High / Flue Gas Sensor Trip
F.26
F.26 is displayed, the boiler does not ignite, and the gas valve does not click.
Gas Valve Regulation Fault / Gas Valve Drive Error
F.27
F.27 error: the boiler attempts to ignite but locks out, sometimes with a smell of gas at the flue.
Flame Detection Fault / Flame Present at Wrong Time
F.28
The boiler will not ignite, multiple ignition attempts are heard, then F.28 is displayed.
Ignition Failure / No Flame Detected
F.29
Boiler lights briefly, then goes out, repeats several times, then displays F.29.
Flame Loss During Operation
F.32
F.32 displayed; the fan does not run or runs erratically, and the boiler does not ignite.
Fan Motor Fault / Fan Speed Error
F.36
F.36 error; the boiler shuts down shortly after ignition and may be accompanied by a flue gas smell outside.
Flue Gas Recirculation / Flue Blockage Detected
F.49
F.49 displayed; the boiler is unresponsive or resets intermittently, indicating a low voltage error.
Low Supply Voltage / 24V DC Fault
F.61
F.61 error; the boiler does not attempt ignition, and the gas valve is not actuated.
Gas Valve Control Fault
F.62
F.62 displayed; the boiler may click repeatedly, resulting in delayed ignition or no ignition.
Gas Valve Shut-off Delay / Gas Valve Sticking
F.63
F.63 error; the boiler is locked out and may occur after a power surge or PCB replacement.
EEPROM / Parameter Memory Fault
F.64
F.64 displayed; the boiler may show abnormal temperature readings or shut down unexpectedly.
Internal Temperature Sensor / PCB Temperature Fault
F.65
F.65 error; the boiler shuts down, and the PCB is very warm to the touch.
Electronics Overtemperature
F.67
F.67 displayed; the boiler fails to ignite or mis-detects the flame.
Flame Detection Electronics Fault
F.70
F.70 error after PCB replacement or parameter change; boiler locked out.
Parameter Set / Appliance Type Mismatch
F.71
F.71 displayed; flow and return temperatures appear identical or unrealistic.
Flow/Return Sensor Plausibility Fault
F.72
F.72 error; boiler may overheat or show inconsistent temperatures.
Flow/Return Temperature Difference Implausible
F.73
F.73 displayed; pressure gauge shows normal, but boiler reports low pressure.
Water Pressure Sensor Open Circuit / Fault
F.74
F.74 error; boiler reports excessively high pressure, and may not start.
Water Pressure Sensor Short Circuit / Implausible High Reading
F.75
F.75 displayed at startup; boiler does not fire, and the pump may run briefly.
No Pressure Rise Detected When Pump Starts
F.76
F.76 error; boiler shuts down, and the primary heat exchanger is very hot.
Primary Heat Exchanger Overtemperature
F.77
F.77 displayed; boiler may start then shut down, and condensate may not drain.
Condensate Pump Fault (on models with integrated pump)
F.83
F.83 error at startup; boiler may not fire or may short-cycle.
Flow Temperature Sensor Temperature Rise Implausible
F.84
F.84 displayed; inconsistent flow temperature readings, and the boiler may lock out.
Flow Sensor / NTC Fault or Contact Problem
F.85
F.85 error: boiler may not start, both NTC sensors reported faulty.
Flow and Return Sensor Fault / Wiring Error
Symptom
Boiler frequently short cycles (turns on and off rapidly) without reaching the set temperature.
Short Cycling / Oversized Boiler or Control Issue
Symptom
Condensate dripping from the bottom of the boiler casing or staining on the wall below the flue.
Condensate Leak / Flue Condensate Backflow
Symptom
Domestic hot water temperature fluctuates or runs hot then cold on the Vaillant combi boiler.
Inconsistent DHW Temperature Control
Symptom
Furnace/boiler leaking water from underneath the casing or from the relief pipe outside.
System Water Leak / Pressure Relief Valve Discharge
Symptom
Loud banging or knocking noises (kettling) from the Vaillant boiler during operation.
Boiler Kettling / Localized Boiling in Heat Exchanger
Symptom
Radiators or underfloor heating not getting hot even though the Vaillant boiler appears to run.
Poor Heating Performance / No Circulation
Symptom
Smell of gas near the Vaillant boiler, even when not running.
Gas Leak at Appliance
Symptom
The Vaillant boiler frequently loses pressure over days or weeks.
Chronic Pressure Loss
Symptom
The boiler display is completely dead, with no lights or response.
No Power / PCB Supply Failure
Symptom
The boiler fan runs, but the burner does not light, and no error code is displayed.
Ignition Sequence Interruption
Symptom
The boiler makes a loud humming or vibrating noise when firing.
Combustion or Mechanical Vibration
Symptom
The boiler will not respond to the room thermostat; heating stays off or on permanently.
Control Signal / Thermostat Communication Fault
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| F.00 |
Boiler/furnace will not start, the display shows F.00, and no ignition attempt is heard.
|
Flow Temperature Sensor Open Circuit | View Details |
| F.01 |
Boiler/furnace locks out with F.01, runs briefly, then shuts down due to overheating.
|
Flow Temperature Sensor Short Circuit | View Details |
| F.10 |
Furnace displays F.10 and will not fire, sometimes after overheating or poor circulation.
|
Return Temperature Sensor Open Circuit | View Details |
| F.11 |
Furnace displays F.11, may overheat quickly or cycle rapidly.
|
Return Temperature Sensor Short Circuit | View Details |
| F.20 |
Boiler shuts down with F.20; casing and flow pipe are very hot, and the pump may be noisy.
|
Safety Temperature Limiter (STB) / Overheat Protection Activated | View Details |
| F.22 |
Display shows F.22, boiler will not fire, and the pressure gauge reads very low or zero.
|
Low Water Pressure / Dry Fire Protection | View Details |
| F.23 |
F.23 is displayed, the boiler cycles off quickly, and the flow temperature rises much faster than the return.
|
Flow/Return Temperature Difference Too High (Poor Circulation) | View Details |
| F.24 |
F.24 error: the boiler overheats rapidly after starting, then shuts down.
|
Low Water Content / Pump Fault / Overheat | View Details |
| F.25 |
Boiler locks out with F.25; the fan may run, but the burner does not stay lit.
|
Flue Gas Temperature Too High / Flue Gas Sensor Trip | View Details |
| F.26 |
F.26 is displayed, the boiler does not ignite, and the gas valve does not click.
|
Gas Valve Regulation Fault / Gas Valve Drive Error | View Details |
| F.27 |
F.27 error: the boiler attempts to ignite but locks out, sometimes with a smell of gas at the flue.
|
Flame Detection Fault / Flame Present at Wrong Time | View Details |
| F.28 |
The boiler will not ignite, multiple ignition attempts are heard, then F.28 is displayed.
|
Ignition Failure / No Flame Detected | View Details |
| F.29 |
Boiler lights briefly, then goes out, repeats several times, then displays F.29.
|
Flame Loss During Operation | View Details |
| F.32 |
F.32 displayed; the fan does not run or runs erratically, and the boiler does not ignite.
|
Fan Motor Fault / Fan Speed Error | View Details |
| F.36 |
F.36 error; the boiler shuts down shortly after ignition and may be accompanied by a flue gas smell outside.
|
Flue Gas Recirculation / Flue Blockage Detected | View Details |
| F.49 |
F.49 displayed; the boiler is unresponsive or resets intermittently, indicating a low voltage error.
|
Low Supply Voltage / 24V DC Fault | View Details |
| F.61 |
F.61 error; the boiler does not attempt ignition, and the gas valve is not actuated.
|
Gas Valve Control Fault | View Details |
| F.62 |
F.62 displayed; the boiler may click repeatedly, resulting in delayed ignition or no ignition.
|
Gas Valve Shut-off Delay / Gas Valve Sticking | View Details |
| F.63 |
F.63 error; the boiler is locked out and may occur after a power surge or PCB replacement.
|
EEPROM / Parameter Memory Fault | View Details |
| F.64 |
F.64 displayed; the boiler may show abnormal temperature readings or shut down unexpectedly.
|
Internal Temperature Sensor / PCB Temperature Fault | View Details |
| F.65 |
F.65 error; the boiler shuts down, and the PCB is very warm to the touch.
|
Electronics Overtemperature | View Details |
| F.67 |
F.67 displayed; the boiler fails to ignite or mis-detects the flame.
|
Flame Detection Electronics Fault | View Details |
| F.70 |
F.70 error after PCB replacement or parameter change; boiler locked out.
|
Parameter Set / Appliance Type Mismatch | View Details |
| F.71 |
F.71 displayed; flow and return temperatures appear identical or unrealistic.
|
Flow/Return Sensor Plausibility Fault | View Details |
| F.72 |
F.72 error; boiler may overheat or show inconsistent temperatures.
|
Flow/Return Temperature Difference Implausible | View Details |
| F.73 |
F.73 displayed; pressure gauge shows normal, but boiler reports low pressure.
|
Water Pressure Sensor Open Circuit / Fault | View Details |
| F.74 |
F.74 error; boiler reports excessively high pressure, and may not start.
|
Water Pressure Sensor Short Circuit / Implausible High Reading | View Details |
| F.75 |
F.75 displayed at startup; boiler does not fire, and the pump may run briefly.
|
No Pressure Rise Detected When Pump Starts | View Details |
| F.76 |
F.76 error; boiler shuts down, and the primary heat exchanger is very hot.
|
Primary Heat Exchanger Overtemperature | View Details |
| F.77 |
F.77 displayed; boiler may start then shut down, and condensate may not drain.
|
Condensate Pump Fault (on models with integrated pump) | View Details |
| F.83 |
F.83 error at startup; boiler may not fire or may short-cycle.
|
Flow Temperature Sensor Temperature Rise Implausible | View Details |
| F.84 |
F.84 displayed; inconsistent flow temperature readings, and the boiler may lock out.
|
Flow Sensor / NTC Fault or Contact Problem | View Details |
| F.85 |
F.85 error: boiler may not start, both NTC sensors reported faulty.
|
Flow and Return Sensor Fault / Wiring Error | View Details |
| – |
Boiler frequently short cycles (turns on and off rapidly) without reaching the set temperature.
|
Short Cycling / Oversized Boiler or Control Issue | View Details |
| – |
Condensate dripping from the bottom of the boiler casing or staining on the wall below the flue.
|
Condensate Leak / Flue Condensate Backflow | View Details |
| – |
Domestic hot water temperature fluctuates or runs hot then cold on the Vaillant combi boiler.
|
Inconsistent DHW Temperature Control | View Details |
| – |
Furnace/boiler leaking water from underneath the casing or from the relief pipe outside.
|
System Water Leak / Pressure Relief Valve Discharge | View Details |
| – |
Loud banging or knocking noises (kettling) from the Vaillant boiler during operation.
|
Boiler Kettling / Localized Boiling in Heat Exchanger | View Details |
| – |
Radiators or underfloor heating not getting hot even though the Vaillant boiler appears to run.
|
Poor Heating Performance / No Circulation | View Details |
| – |
Smell of gas near the Vaillant boiler, even when not running.
|
Gas Leak at Appliance | View Details |
| – |
The Vaillant boiler frequently loses pressure over days or weeks.
|
Chronic Pressure Loss | View Details |
| – |
The boiler display is completely dead, with no lights or response.
|
No Power / PCB Supply Failure | View Details |
| – |
The boiler fan runs, but the burner does not light, and no error code is displayed.
|
Ignition Sequence Interruption | View Details |
| – |
The boiler makes a loud humming or vibrating noise when firing.
|
Combustion or Mechanical Vibration | View Details |
| – |
The boiler will not respond to the room thermostat; heating stays off or on permanently.
|
Control Signal / Thermostat Communication Fault | View Details |
No matching codes found
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