Vaillant Boiler Issues: Errors and Fault Codes

There might be times when your Vaillant boiler displays a fault code but this won’t necessarily mean that your boiler is to blame for the error. It could be that your Vaillant boiler just needs resetting or there is a problem that can be easily fixed, such as the boiler is losing pressure and simply needs re-pressurising. By understanding what the fault codes mean, you can quickly determine whether or not you need to call in an engineer. If it’s a complex fault that requires repairs, you must contact a qualified engineer and get them to carry out the work.

If a fault code has appeared on your Vaillant boiler, you can scroll through the table below to find out what the code means.

Vaillant Boiler Fault Codes

FAULT CODE DESCRIPTION / POSSIBLE CAUSE
F.00 Interruption/Flow sensor. NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness or NTC defective.
F.01 Interruption/Return sensor. NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness or NTC defective.
F.02 Interruption/DHW outlet sensor. Only in conjunction with F.91 NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics.
F.03 Interruption/Cylinder sensor. Only in conjunction with F.91 NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics
F.10 Short circuit/Flow sensor. NTC defective, short circuit in the cable harness, cable or casing.
F.11 Short circuit/Return sensor. NTC defective, short circuit in the cable harness, cable or casing.
F.12 Short circuit/DHW outlet sensor. Only in conjunction with F.91 NTC defective, short circuit in the cable harness, cable or casing.
F.13 Short circuit/Cylinder sensor/Combination product. Warm start sensor/cylinder sensor short circuit, combination product with actoSTOR. Short circuit cylinder sensor (NTC) only in combination with F.91 NTC defective, short circuit in cable harness, cable or casing.
F.20 Safety switch off. Temperature limiter. Incorrect earth connection between cable harness and product, flow or return NTC defective (loose connection), black discharge via ignition cable, ignition plug or ignition electrode.
F.22 Safety switch off. Low water pressure. No or insufficient water in the boiler or water pressure sensor defective.
F.23 Safety switch off. Temperature difference too large. Pump blocked, insufficient pump output, air in boiler or flow and return NTC sensor connected the wrong way round.
F.24 Safety switch off. Temperature increasing too fast. Pump blocked, insufficient pump output, air in boiler, system pressure too low or non-return valve blocked or incorrectly installed.
F.25 Safety switch off. Flue temperature too high. Break in plug connection for optional flue gas safety temperature limiter (STB) or break in cable harness.
F.26 Fuel valve not working. Gas valve stepper motor not connected, multiple plug on the PCB not plugged in correctly, interruption in cable harness, gas valve stepper motor defective or electronics defective.
F.27 Safety switch off. Flame simulation. Moisture on the electronics, electronics flame monitor defective or gas solenoid valve is leaking.
F.28 Start-up failure/Ignition unsuccessful. Gas meter defective or gas pressure monitor has triggered, air in gas, gas flow pressure too low, thermal isolator device (TAE) has triggered, condensate duct blocked, incorrect gas restrictor, incorrect spare gas valve, fault on gas valve, multiple plug on PCB incorrectly plugged in, break in cable harness, ignition system defective, ionisation current interrupted, incorrect earthing of product or electronics defective.
F.29 Start-up failure/Ignition unsuccessful. Gas supply temporarily stopped, flue gas recirculation, condensate duct blocked, incorrect earthing of product or ignition transformer has spark failure.
F.32 Fault with the fan. Plug on fan not plugged in correctly, multiple plug on PCB not plugged in correctly, break in cable harness, fan blocked, hall sensor defective or electronics defective.
F.49 eBUS fault. Short circuit on eBUS, eBUS overload or two power supplies with different polarities on the eBUS.
F.52 Contact fault mass flow sensor/Venturi. The mass flow sensor/Venturi is not connected electronically, the plug is not plugged in correctly, the plug is not plugged in, the plug is defective, the slot is defective or the mass flow sensor/Venturi is defective.
F.53 Combustion regulation fault. Gas flow pressure too low, liquid gas coding resistance used when operating with natural gas. If the fault occurs again after being cleared, the gas valve is defective, the mass flow sensor/Venturi is defective, wet or blocked. If the fault occurs again after being cleared, do not wet the sensor and do not use any lubricants on the O-ring on the Venturi.
F.54 Gas supply fault (in combination with F.28/F.29). There is insufficient gas supply to operate the unit, the gas isolator cock(s) is closed, the gas flow pressure is too low or the gas valve is defective.
F.56 Safety switch off/CO limit exceeded. Defective component in the combustion regulation, contact fault at the gas valve (not plugged in correctly, not plugged in, plug defective or slot is defective). If the fault occurs again after being cleared, the gas valve is defective.
F.57 Measuring program fault. Active comfort protection mode has detected a regulation fault. Ignition electrode highly corroded.
F.61 Fuel valve actuation fault. The gas valve cannot be actuated, cable harness supply line to the gas valve is defective (short to earth, short circuit), gas valve defective or PCB defective.
F.62 Fuel valve switch-off delay. Delayed shutdown of gas valve detected. Secondary light (ignition and monitoring electrode indicates delayed extinguishing of the flame signal), gas valve defective or PCB defective.
F.63 EEPROM fault. Electronics defective.
F.64 Electronics/Sensor fault. Flow or return NTC short circuited or electronics defective.
F.65 Electronics temperature fault. Electronics overheating due to external influences, electronics defective.
F.67 Electronics/Flame fault. Implausible flame signal, electronics defective.
F.68 Flame signal unstable. Air in gas, gas flow pressure too low, incorrect air ratio, condensate duct blocked, ionisation flow interruption (cable, electrode), flue gas recirculation or condensate duct.
F.70 Invalid Device Specific Number (DSN). If spare parts fitted – Display and PCB replaced at the same time and DSN not reset, incorrect or missing output range coding resistance.
F.71 Fault with flow/return sensor. Flow temperature sensor signalling constant value, flow temperature sensor incorrectly positioned on supply pipe or flow temperature sensor defective.
F.72 Fault with flow/return sensor. Flow/return NTC temperature difference too great. Flow and/or return temperature sensor is defective.
F.73 Water pressure sensor fault. Interruption/short circuit of water pressure sensor, interruption/short circuit to GND in supply line to water pressure sensor or water pressure sensor defective.
F.74 Water pressure sensor fault. The line to the water pressure sensor has a short circuit to 5 V/24 V or internal fault in the water pressure sensor.
F.75 Pump fault/Water shortage. Water pressure sensor and/or pump defective, air in the heating installation, insufficient water in the product, connect external expansion vessel to the return.
F.77 No response from flue non-return flap or condensate pump defective.
F.78 DHW outlet sensor on ext. contr. UK link box is connected but the hot water NTC is not bridged.
F.80 Fault with actoSTOR inlet sensor. Only in conjunction with F.91 NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics. Sensor plug has short to earth to the casing, short circuit in the cable harness or sensor defective.
F.81 Fault with cylinder charging pump. Only in conjunction with F.91 cylinder is not fully charged after specified time. Check cylinder charging sensor and cylinder sensor (air in the actoSTOR pump). Inspect cable harness for pump. Check the impeller sensor and/or limiter in the product. Diverter valve defective, secondary heat exchanger blocked or pump faulty.
F.83 Fault with burner temperature sensor. When burner starts, the temperature change registered at the flow and/or return temperature sensor is non-existent or too small. Insufficient water in product or the flow or return temperature is in the wrong position on the pipe.
F.84 Fault with flow/return temperature sensor. Flow and return temperature sensors returning implausible values. Flow and return temperature sensors have been inverted or have not been correctly installed.
F.85 NTC fitted incorrectly. The flow and/or return temperature sensors have been installed on the same pipe or incorrect pipe.
F.90 Communication fault. Check the cable harness from the product to the actoSTOR module (PEBus). If the product is to be operated without an actoSTOR module, setD.092 = 0.
F.91 Sensor/Actuator fault on actoSTOR module.
F.92 Coding resistance fault. The coding resistance on the PCB does not match the entered gas group. Check the resistance, repeat the gas family check and enter the correct gas group.
F.93 Poor combustion quality. Combustion regulation has detected poor combustion quality. Incorrect gas restrictor has been installed for the gas family (another gas family is present). Mass flow sensor/Venturi defective (wet or blocked). Do not wet the sensor and do not use any lubricants on the O-ring on the Venturi.
LED actoSTOR module actoSTOR electronics status. LED on. Communication OK LED flashing. Communication not OK LED off. No power supply.
Err Communication fault between the display and PCB in the electronics box.