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Vaillant Boiler Fault Codes

Boasting German design and engineering, Vaillant boilers are about as reliable and impressive as they come but that’s not to say that things can’t go a bit wrong every now and then. Boilers are no different from any other type of appliance in that they can encounter faults throughout their life, no matter their build quality and how well you look after them. But one of the good things about Vaillant boilers is that they actually tell you when they’re in trouble by displaying special fault codes.

The tables below give information about Vaillant boiler fault codes and error messages to help you understand what each code means when they appear on your boiler.

Types of Vaillant Boiler Fault Codes

FAULT CODEPOSSIBLE CAUSE
F.00INTERRUPTION TO FLOW SENSOR – NTC PLUG NOT PLUGGED IN OR HAS COME LOOSE, MULTIPLE PLUG ON PCB INCORRECTLY PLUGGED IN, INTERRUPTION IN CABLE HARNESS OR NTC DEFECTIVE
F.01INTERRUPTION TO RETURN SENSOR – NTC PLUG NOT PLUGGED IN OR HAS COME LOOSE, MULTIPLE PLUG ON PCB INCORRECTLY PLUGGED IN, INTERRUPTION IN CABLE HARNESS OR NTC DEFECTIVE
F.02INTERRUPTION TO DHW OUTLET SENSOR – NTC DEFECTIVE, NTC CABLE DEFECTIVE, DEFECTIVE PLUG CONNECTION IN NTC OR DEFECTIVE PLUG CONNECTION ON THE ASTOSTOR ELECTRONICS
F.03INTERRUPTION TO CYLINDER SENSOR - NTC DEFECTIVE, NTC CABLE DEFECTIVE, DEFECTIVE PLUG CONNECTION IN NTC OR DEFECTIVE PLUG CONNECTION ON THE ASTOSTOR ELECTRONICS
F.10FLOW SENSOR SHORT CIRCUIT – NTC DEFECTIVE OR SHORT CIRCUIT IN CABLE HARNESS, CABLE OR CASING
F.11RETURN SENSOR SHORT CIRCUIT - NTC DEFECTIVE OR SHORT CIRCUIT IN CABLE HARNESS, CABLE OR CASING
F.12DHW OUTLET SENSOR SHORT CIRCUIT - NTC DEFECTIVE OR SHORT CIRCUIT IN CABLE HARNESS, CABLE OR CASING
F.13CYLINDER SENSOR SHORT CIRCUIT / COMBINATION PRODUCT / WARM START SENSOR OR CYLINDER SENSOR SHORT CIRCUIT COMBINATION PRODUCT WITH ACTOSTOR – SHORT CIRCUIT CYLINDER SENSOR (NTC) DEFECTIVE OR SHORT CIRCUIT IN CABLE HARNESS, CABLE OR CASING
F.20SAFETY SWITCH-OFF: TEMPERATURE LIMITER – INCORRECT EARTH CONNECTION BETWEEN CABLE HARNESS AND PRODUCT, DEFECTIVE FLOW OR RETURN NTC, BLACK DISCHARGE VIA IGNITION CABLE OR IGNITION PLUG OR ELECTRODE
F.22SAFETY SWITCH-OFF: LOW WATER PRESSURE – INSUFFICIENT OR NO WATER IN BOILER OR DEFECTIVE WATER PRESSURE SENSOR
F.23SAFETY SWITCH-OFF: TEMPERATURE DIFFERENCE TOO LARGE – BLOCKED PUMP, INSUFFICIENT PUMP OUTLET, AIR IN BOILER OR FLOW AND RETURN NTC CONNECTED WRONG WAY ROUND
F.24SAFETY SWITCH-OFF: TEMPERATURE INCREASING TOO FAST – BLOCKED PUMP, INSUFFICIENT PUMP OUTLET, AIR IN BOILER, SYSTEM PRESSURE TOO LOW OR NON-RETURN VALVE BLOCKED OR INCORRECTLY INSTALLED
F.25SAFETY SWITCH-OFF: FLUE TEMPERATURE TOO HIGH – BROKEN PLUG CONNECTION FOR OPTIONAL FLUE GAS SAFETY TEMPERATURE LIMITER OR BROKEN CABLE HARNESS
F.26FUEL VALVE FAULT: GAS VALVE STEPPER MOTOR NOT CONNECTED, MULTIPLE PLUG ON PCB INCORRECTLY PLUGGED IN, INTERRUPTION IN CABLE HARNESS, DEFECTIVE GAS VALVE STEPPER OR DEFECTIVE ELECTRONICS
F.27SAFETY SWITCH-OFF: FLAME SIMULATION – MOISTURE IN ELECTRONICS, LEAKING GAS SOLENOID VALVE OR DEFECTIVE FLAME MONITOR
F.28START-UP FAILURE: DEFECTIVE GAS METER, GAS PRESSURE MONITOR HAS TRIGGERED, AIR IN GAS, GAS FLOW PRESSURE TOO LOW, TAE HAS TRIGGERED, BLOCKED CONDENSATE DUCT, INCORRECT GAS RESTRICTOR, INCORRECT GAS VALVE, FAULTY GAS VALVE, BROKEN CABLE HARNESS MULTIPLE PLUG ON PCB INCORRECTLY PLUGGED IN, INCORRECT EARTHING OF PRODUCT OR DEFECTIVE ELECTRONICS
F.29START-UP FAILURE: GAS SUPPLY STOPPED, FLUE GAS RECIRCULATION, BLOCKED CONDENSATE DUCT, INCORRECT EARTHING OF PRODUCT OR IGNITION TRANSFORMER SPARK FAILURE
F.32FAN FAULT: FAN OR PLUG INCORRECTLY PLUGGED IN, MULTIPLE PLUG ON PCB INCORRECTLY PLUGGED IN, BROKEN CABLE HARNESS, BLOCKED FAN OR DEFECTIVE ELECTRONICS OR HALL SENSOR
F.49EBUS FAULT: EBUS SHORT CIRCUIT, EBUS OVERLOAD OR 2 POWER SUPPLIES WITH DIFFERENT POLARITIES ON EBUS
F.52FAULT WITH MASS FLOW SENSOR CONNECTION: MASS FLOW SENSOR NOT CONNECTED, PLUG INCORRECTLY PLUGGED IN OR DEFECTIVE PLUG
F.53FAULT WITH COMBUSTION REGULATION: GAS FLOW PRESSURE TOO LOW, DEFECTIVE GAS VALVE, DEFECTIVE MASS FLOW SENSOR OR WET OR BLOCKED MASS FLOW SENSOR
F.54GAS SUPPLY FAULT: INSUFFICIENT GAS SUPPLY TO OPERATE BOILER, GAS VALVE DEFECTIVE OR GAS PRESSURE TOO LOW
F.56SAFETY SWITCH-OFF: CO LIMIT EXCEEDED – DEFECTIVE COMPONENT IN COMBUSTION REGULATION, CONTACT FAULT AT GAS VALVE
F.57MEASURING PROGRAM FAULT: FAULT DETECTED BY ACTIVE COMFORT PROTECTION MODE OR HIGHLY CORRODED IGNITION ELECTRODE
F.61FUEL VALVE ACTUATION FAULT: DEFECTIVE PCB, GAS VALVE OR CABLE HARNESS
F.62FUEL VALVE SWITCH-OFF DELAY FAULT: DEFECTIVE PCB, DEFECTIVE GAS VALVE, DELAYED SHUTDOWN OF GAS VALVE OR DELAYED EXTINGUISHING OF FLAME SIGNAL
F.63EEPROM FAULT: DEFECTIVE ELECTRONICS
F.64ELECTRONICS/NTC FAULT: FLOW OR RETURN NTC SHORT CIRCUITED – DEFECTIVE ELECTRONICS
F.65ELECTRONIC TEMPERATURE FAULT: ELECTRONICS OVERHEATED – DEFECTIVE ELECTRONICS
F.67ELECTRONICS/FLAME FAULT: IMPLAUSIBLE FLAME SIGNAL – DEFECTIVE ELECTRONICS
F.68FLAME SIGNAL UNSTABLE: AIR IN GAS, GAS FLOW PRESSURE TOO LOW, INCORRECT AIR RATIO, BLOCKED CONDENSATE DUCT, IONISATION FLOW INTERRUPTION OR FLUE GAS RECIRCULATION
F.70INVALID DEVICE SPECIFIC NUMBER FAULT: INCORRECT OR MISSION OUTPUT RANGE CODING RESISTANCE – DISPLAY AND PCB HAVE BEEN REPLACED AT THE SAME TIME BUT DSN HAS NOT BEEN RESET
F.71FLOW SENSOR FAULT: FLOW TEMPERATURE SENSOR DEFECTIVE OR INCORRECTLY PLACED AT SUPPLY PIPE
F.72FLOW/RETURN SENSOR FAULT: FLOW/RETURN TEMPERATURE DIFFERENCE TOO GREAT OR FLOW AND/OR RETURN TEMPERATURE SENSOR DEFECTIVE
F.73WATER PRESSURE SENSOR FAULT: INTERRUPTION OR SHORT CIRCUIT OF WATER PRESSURE SENSOR OR TO GND IN SUPPLY LINE TO WATER PRESSURE SENSOR OR DEFECTIVE WATER PRESSURE SENSOR
F.74WATER PRESSURE SENSOR FAULT: LINE TO WATER PRESSURE HAS A SHORT CIRCUIT TO 5V/24V OR THERE IS AN INTERNAL FAULT IN THE WATER PRESSURE SENSOR
F.75PUMP/WATER SHORTAGE FAULT: DEFECTIVE PUMP AND/OR WATER PRESSURE SENSOR, AIR IN THE HEATING INSTALLATION OR INSUFFICIENT WATER IN BOILER – CONNECT EXTERNAL EXPANSION VESSEL TO THE RETURN
F.77FLUE NON-RETURN VALVE/CONDENSATE PUMP FAULT: NO RESPONSE FROM FLUE NON-RETURN FLAP OR DEFECTIVE CONDENSATE PUMP
F.78INTERRUPTION TO DHW OUTLET SENSOR OR EXTERNAL CONTROLLER: UK LINK IS CONNECTED BUT HOT WATER NTC IS NOT BRIDGED
F.80ACTOSTOR INLET SENSOR FAULT: DEFECTIVE NTC, NTC CABLE, PLUG CONNECTION ON NTC OR PLUG CONNECTION ON THE ACTOSTOR ELECTRONICS
F.81CYLINDER CHARGING PUMP FAULT: CYLINDER NOT FULLY CHARGED AFTER SPECIFIED TIME (CHECK CYLINDER CHARGING SENSOR AND CYLINDER SENSOR), AIR IN ACTOSTOR PUMP (INSPECT CABLE HARNESS FOR PUMP), DEFECTIVE DIVERTER VALVE, FAULT PUMP OR BLOCKED SECONDARY HEAT EXCHANGER
F.83FLOW AND/OR RETURN TEMPERATURE SENSOR FAULT: THE TEMPERATURE CHANGE REGISTERED AT THE FLOW AND/OR RETURN TEMPERATURE SENSOR IS TOO SMALL OR NON-EXISTENT – INSUFFICIENT WATER IN BOILER OR FLOW OR TEMPERATURE SENSOR INCORRECTLY POSITIONED AT PIPE
F.84FLOW/RETURN TEMPERATURE DIFFERENCE IMPLAUSIBLE: IMPLAUSIBLE VALUES FOR FLOW AND RETURN TEMPERATURE SENSORS HAVE BEEN DETECTED – FLOW AND RETURN TEMPERATURE SENSORS HAVE BEEN INVERTED OR INCORRECTLY INSTALLED
F.85FLOW AND/OR RETURN TEMPERATURE SENSOR INSTALL FAULT: THE FLOW AND/OR RETURN TEMPERATURE SENSORS ARE INSTALLED ON THE SAME OR WRONG PIPE
F.90COMMUNICATION FAULT: NEED TO CHECK THE CABLE HARNESS FROM THE BOILER TO THE ACTOSTOR MODULE
F.91ACTOSTOR SENSOR/ACTUATOR FAULT
F.92CODING RESISTANCE FAULT: CODING ON PCB DOES NOT MATCH THE ENTERED GAS GROUP – CHECK RESISTANCE, REPEAT THE GAS GROUP CHECK AND ENTER THE CORRECT GAS GROUP
F.93COMBUSTION QUALITY FAULT: POOR COMBUSTION QUALITY – WRONG GAS RESTRICTOR HAS BEEN INSTALLED FOR THE GAS GROUP, RECIRCULATION, WET OR BLOCKED MASS FLOW SENSOR/VENTURI (DO NOT WET SENSOR OR USE ANY LUBRICANTS ON THE VENTURI O-RING)

If your Vaillant boiler is currently showing a fault code that isn’t listed on this page, you can find more information via the Vaillant website. You can also find a troubleshooting guide on the website but if you need further help solving any problems, we recommend you get in touch with Vaillant or a qualified Gas Safe registered engineer.

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