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