It’s important to note that these fault codes may vary depending on the specific model and version of the Vaillant boiler. Always refer to the boiler’s manual or consult a qualified heating engineer for accurate diagnosis and repair.
Vaillant F.00 & F.01 Error Fault Codes
Interruption to Heating Flow Temperature Sensor or Heating Flow NTC Thermistor fault.
Reason(s):
- Unplugged NTC plug
- Loose NTC plug
- Multiple plugs on the PCB not plugged in correctly
- Interruption in cable harness
- Defective NTC
F.00 & F.01 Error Fix
The Heating Flow Thermistor cable or wiring harness will need to be replaced or the NTC itself will need to be replaced.
Vaillant F.02 Boiler Error Code
Interruption to DHW outlet sensor in conjunction with F.91 error.
Reason(s):
- Defective NTC
- Defective NTC cable
- Defective NTC plug connection
- Defective actoSTOR electronics plug connections
F.02 Error Fix
The Heating Flow Thermistor cable or wiring harness will need to be replaced or the NTC itself will need to be replaced.
Vaillant F.03 Boiler Error Code
Interruption to the cylinder sensor actoSTOR (NTC) in conjunction with F.91.
Reason(s):
- Defective NTC
- Defective NTC cable
- Defective NTC plug connection
- Defective actoSTOR electronics plug connections
F.03 Error Fix
The Heating Flow Thermistor cable or wiring harness will need to be replaced or the NTC itself will need to be replaced.
Vaillant F.10 Boiler Error Code
Short circuit in Flow NTC fault
Reason(s):
- Defective NTC
- Short circuit in cable harness, cable or casing
F.10 Error Fix
The Heating Flow Thermistor cable or wiring harness will need to be replaced or the NTC itself will need to be replaced.
Vaillant F.11 Boiler Error Code
Heating Flow NTC Thermistor Fault or Heating Return NTC Short Circuit
Reason(s):
- Short circuit on the Heating Return NTC Thermistor (should read 12Kohms @ 20C)
- Wiring harness fault
- Heating Return NTC Thermistor and Main PCB connection error
F.11 Error Fix
Gas Safe engineer is required to check the NTC sensor and wiring using a continuity tester and resistance tester. NTC, harness, or PCB replacement may be necessary.
Vaillant F.12 Boiler Error Code
Short circuit in cylinder charging sensor (NTC) in combination with F.91
Reason(s):
- Mass short in sensor plug to casing
- Short circuit in the wiring loom
- Fault sensor
F.12 Error Fix
NTC sensor plug, wiring loom, and sensor should be tested for errors by a gas-safe engineer.
Vaillant F.13 Boiler Error Code
- VU-/VUW boiler: Short circuit warm start sensor/cylinder sensor
- VUW boiler with actoSTOR: Short circuit cylinder sensor (NTC)
In combination with F.91/
Reason(s):
- Defective NTC
- Short circuit in cable harness, cable, or casing
F.13 Error Fix
The Heating Flow Thermistor cable or wiring harness will need to be replaced or the NTC itself will need to be replaced. All of these solutions will require a gas-safe engineer.
Vaillant F.20 Boiler Error Code
Safety Switch-Off: Temperature Limiter.
Reason(s):
The boiler has overheated and must cool down.
F.20 Error Fix
Turn off your boiler and allow it to cool down before turning back on again. If the problem persists please contact a Gas Safe engineer.
Vaillant F.22 Boiler Error Code
Safety Switch-off: low water pressure.
Reason(s):
- No water pressure
- Pressure too low in the boiler
- Defective water pressure sensor
- Loose, Disconnected, or defective cable to pump
F.22 Error Fix
If you have low or no water pressure, top up the pressure of the boiler using the internal filling loop for combi boilers or the external filling loop for system boilers.
Vaillant F.23 Boiler Fault Code
Safety switch-off: Temperature difference too great.
Reason(s):
- Inadequate pump operation
- Blockage or air lock within the appliance
- Blockage or air lock within the heating system
F.23 Error Fix
Call out a Gas Safe engineer to check the flow and return NTC sensors and wiring with a continuity tester and resistance tester if necessary. The NTC, harness or PCB may need to be replaced depending on the results of the checks.
They should also ensure there are no blockages in the pipework and conduct a power flush if there is a high amount of debris.
Vaillant F.24 Boiler Fault Code
Water circulation fault (temperature rise greater than 10ºC per second).
Reason(s):
- Inadequate pump operation
- Isolation valves closed under the appliance
- Blockage or air lock within the appliance
- Blockage or air lock within the heating system
F.24 Error Fix
Call out a Gas Safe engineer to check the flow and return NTC sensors and wiring with a continuity tester and resistance tester if necessary. The NTC, harness, or PCB may need to be replaced depending on results of checks.
They should also ensure there are no blockages in the pipework and conduct a power flush if there is a high amount of debris.
Vaillant F.25 Boiler Fault Code
Safety shut-off: flue gas temperature is too high.
Reason(s):
- Flue gas temperature limiter STB is faulty
- Flue gas temperature limiter STB is sooted up
- Broken plug
- Fault cable harness
F.25 Error Fix
Call out a Gas Safe engineer to check the STB, clean and retest. If the issue persists, the limit may need replacing. Test plug and cable harness for issues.
Vaillant F.27 Boiler Fault Code
Safety switch-off: Flame simulation. Incorrect flame detection fault
Reason(s):
- Damaged electrode or ignition lead
- Moisture on electronics
- Spark generator fault
- Main PCB fault
F.27 Error Fix
Call out a Gas Safe engineer to check for any moisture on the electronic or PCB and test the spark generator, electrode, or ignition lead. If any issues are detected, they may need to be replaced.
Vaillant F.28 Boiler Fault Code
Failure during start-up: unsuccessful ignition.
Reason(s):
- Defective gas meter or gas pressure regulator
- Air in gas
- Gas flow pressure too low
- Triggered thermal isolator device (TAE)
- Blocked condensate duct
- Incorrect gas restrictor
- Incorrect ET gas valve
- Fault on the ET gas valve
- Multiple PCB plus incorrectly plugged-in
- Cable harness breakage
- Defective ignition system (transformer, cable, plug, or electrode)
F.28 Error Fix
Ensure gas is coming to your property at the mains. If it isn’t, make sure to contact your gas supplier. If it is, make sure to call out a Gas Safe engineer to test your system and identify the fault.
Vaillant F.29 Boiler Fault Code
Operation failure: re-ignition unsuccessful.
Reason(s):
- Temporary stop in gas supply
- Flue gas recirculation
- Blocked condensate duct
- Defective earthing of product
- Spark failure in ignition transformer
F.29 Error Fix
Ensure gas is coming to your property at the mains. If it isn’t, make sure to contact your gas supplier. If it is, make sure to call out a Gas Safe engineer to test your system and identify the fault.
Vaillant F.32 Boiler Fault Code
Fan Fault
Reason(s):
- Fan plug incorrectly plugged in
- Multiple plug on PCB incorrectly plugged in
- Broken cable harness
- Blocked fan
- Defective hall sensor
- Defective electronics
F.32 Error Fix
Call out a Gas Safe engineer to investigate and fix the problem.
Vaillant F.35 Boiler Fault Code
Air/flue gas duct fault
Reason(s):
Blocked flue.
F.35 Error Fix
Call out a Gas Safe engineer to check for any blockages either internally or externally to the flue or the fan.
Vaillant F.42 Boiler Fault Code
Short circuit or interruption in coding resistor output range located in the cable harness or gas group resistor.
Reason(s):
Issue between the coding resistor and PCB, sometimes in combination with (or as a result of) F.70.
F.42 Error Fix
Call out a Gas Safe engineer to check the connections between the coding resistor and PCB.
Vaillant F.49 Boiler Fault Code
eBUS fault
Reason(s):
- eBUS short circuit
- eBUS overload
- Two power supplies have different polarities on the eBUS
F.49 Error Fix
This issue requires a Gas Safe engineer.
Vaillant F.52 and F.53 Boiler Fault Codes
There is an issue with the mass flow sensor.
Reason(s):
- Sensor connection fault
- Faulty flow sensor function
- Wet filter cap
- Blocked filter cap
- Gas flow pressure too low
F.52 / F.53 Error Fix
Make sure the sensor is connected properly by removing it and plugging it back in. This typically fixes the F.52 error.
If this doesn’t solve the issue, you may need a Gas Safe engineer to come out and electronically test the functioning of the sensor and its output valves. If there is an issue with the function, the sensor will likely need replacing.
Alternatively, there may be an issue with the gas supply pressure, in which case your gas supplier will likely need to fix a problem on their end.
Vaillant F.54 Boiler Fault Code
Fault in the gas supply. Often accompanied by an F.28 or F.29 error code.
Reason(s):
- Insufficient gas supply
- Closed gas isolator on boiler or elsewhere
- Gas flow pressure too low
F.54 Error Fix
Make sure your gas stop cock is fully open at the meter, near the boiler, or anywhere else where they may be other stop cocks.
If this doesn’t fix the issue, you should call out a Gas Safe engineer to test the gas valve and replace it if necessary.
Vaillant F.56 Boiler Fault Code
Mass flow sensor regulation fault.
Reason(s):
- Defective gas valve
- Defective cable harness to gas valve
F.56 Error Fix
Call out a Gas Safe engineer to test the gas valve and replace it if necessary.
Vaillant F.57 Boiler Fault Code
Fault during comfort safety mode.
Reason(s):
Highly corroded ignition electrode.
F.57 Error Fix
Call out a Gas Safe engineer to inspect the ignition electrode and replace it as necessary.
Vaillant F.61 Boiler Fault Code
Gas valve actuation fault.
Reason(s):
- Defective cable harness supply line to gas valve
- Defective gas valve
- PCB defective
F.61 Error Fix
Call a Gas Safe engineer to check the resistance across the gas valve for 50-54 ohms (+/-10%). If this is correct, the PCB likely needs to be replaced.
If the gas valve fails the resistance test, the gas valve will likely need replacing.
Vaillant F.62 Boiler Fault Code
Gas valve switch-off delay.
Reason(s):
- Delayed shutdown of gas valve
- Defective gas valve
- Defective PCB
F.62 Error Fix
Call a Gas Safe engineer to test the local tightness of the gas valve. If the gas is tight, you may need the PCB or gas valve replaced.
Vaillant F.63 Boiler Fault Code
Problem with the circuit board (PCB).
Reason(s):
- Old age – general wear and tear
- Loose or damaged connections to and from the PCB
- Incorrectly set-up PCB following installation or service
F.63 Error Fix
This will usually require a replacement PCB, however, this can cost between £350 to £450, so it may be worth replacing your entire boiler if it is getting on a bit or encountering lots of issues.
Vaillant F.64 Boiler Fault Code
An issue with the electrics:
- Faulty NTC connections
- Fault PCB
Reason(s):
- Short-circuit in flow
- Return NTC
- Defective electronics
F.64 Error Fix
Call out a Gas Safe engineer to test your electronics and fix the issue.
Vaillant F.65 Boiler Fault Code
The main PBC temperature is too high.
Reason(s):
- Problem with the mains input voltage
- Loose or damaged electrical connection on the main PCB
- Main PCB fault
F.65 Error Fix
Call out a Gas Safe engineer to test your electronics and fix the issue.
Vaillant F.67 Boiler Fault Code
Flame Signal Fault on Main PCB.
Reason(s):
- Loose or damaged electrical connection on the main PCB
- Faulty Main PCB
- Problem with the Mains Input Voltage (reverse polarity, poor earth, etc)
F.67 Error Fix
Call out a Gas Safe engineer to test your electronics and fix the issue.
Vaillant F.68 Boiler Fault Code
Unstable flame signal
Reason(s):
- Insufficient inlet working gas pressure to the appliance
- Problem with the flue
- Incorrect gas valve adjustment
- Debris in the heat engine
- Faulty electrode or ignition lead
- Fault spark generator
- Faulty gas valve
F.68 Error Fix
Due to the high number of possible causes for this error code, it is best to call out a Gas Safe engineer to test your electronics and fix the issue
Vaillant F.70 Boiler Fault Code
Invalid device-specific number (DSN).
Reason(s):
The most common cause of this error code is when the PCB has not been assigned its software identification for the model of boiler it is intended for. Without this number, it cannot operate safely and therefore produces an F.70 error code.
F.68 Error Fix
Call out a Gas Safe engineer to test your PCB to check whether the code is caused by a malfunction or if the DSN can be assigned through another interface.
Vaillant F.71 Boiler Fault Code
Disconnected or defective heating flow NTC thermistor.
Reason(s):
- Heating flow NTC thermistor isn’t connected to the flow pipe on the main heat exchanger correctly
- Defective heating flow NTC thermistor
F.71 Error Fix
Call out a Gas Safe engineer to check whether the wiring has become disconnected. If it has, it will simply need reconnecting but if it hasn’t then there may be a damaged wire that is causing intermittent signals to the PCB which will need fixing.
Vaillant F.72 Boiler Fault Code
Permanent temperature difference between the flow and return NTC thermistors.
Reason(s):
- Incorrect resistance on the flow and return thermistors
- Wiring issue between the NTC thermistor and the PCB
- Thermistors have been fitted to incorrect pipes during installation
F.72 Error Fix
Call out a Gas Safe engineer to check if any wiring has become disconnected. If it hasn’t you will likely need to have your thermistor replaced.
Vaillant F.73 and F.74 Boiler Fault Codes
The water pressure sensor is too low.
Reason(s):
- Loose, damaged, or broken connection between the sensor and PCB
- The sensor and main PCB connection have water damage
- Faulty pressure sensor
F.73 / F.74 Error Fix
Call out a Gas Safe engineer to check for issues with the wiring and water pressure sensor.
Vaillant F.75 Boiler Fault Code
No pressure change detection when starting the pump.
Reason(s):
The most common cause for the F.75 error code is where a blockage has caused a component to fail. If one component has lots of sludge inside, this is a good indicator that this is the blocked component.
F.75 Error Fix
Call out a Gas Safe engineer to test your system for blockages. Your system will then likely require a power flush or a replacement pump or pump seals.
Vaillant F.76 Boiler Fault Code
Overheating protection on the primary heat exchanger has been triggered.
Reason(s):
- Fused cable or cable connections for safety in primary heat exchanger
- Defective primary heat exchanger
F.76 Error Fix
Call out a Gas Safe engineer to check the connections leading to the heat exchanger. If the heat exchanger is broken or damaged, it is a very expensive job to fix or replace, so your best bet is to replace the boiler entirely.
Vaillant F.77 Boiler Fault Code
Fault with the flue non-return flap or condensate pump.
Reason(s):
Condensate pump blockage or malfunction.
F.77 Error Fix
Call out a Gas Safe engineer to check the condensate pump for a blockage or fault. If there is a blockage, it will need to be cleared. If there is no blockage, a new condensate pump will be required.
Vaillant F.83 Boiler Fault Code
Flow and/or return temperature sensor fault.
Reason(s):
- Temperature change registered at flow an/or return sensor is non-existent or too small
- Not enough water in the boiler
- Flor and/or return temperature sensor not in the correct position at the pipe
F.83 Error Fix
Due to the high number of possible causes for the F.83 error code, you will need to call out a Gas Safe engineer to diagnose the problem and fix it.
Vaillant F.84 Boiler Fault Code
Permanent temperature difference on the NTC flow/return thermistors.
Reason(s):
- Flow and return temperature sensors return implausible values
- Flow and return temperature sensors are inverted
- Flow and return temperature sensors are incorrectly fitted
F.84 Error Fix
Call a Gas Safe engineer to check the thermistors are installed correctly and working. They should also check the PCB to ensure there isn’t a malfunction when reading the thermistors.
Vaillant F.85 Boiler Fault Code
Flow and return temperature sensors are incorrectly fitted.
Reason(s):
Flow and return temperature sensors have been fitted to the same pipe or the wrong pipe.
F.85 Error Fix
Call out a Gas Safe engineer to manually check the reading for both temperature and resistance and either reinstall or replace the thermistors.
Vaillant F.92 Boiler Fault Code
Coding resistor fault
Reason(s):
The coding resistor on the PCB doesn’t match the entered gas family.
F.92 Error Fix
Call out a Gas Safe engineer to check the resistor and enter the correct gas family.
Vaillant F.93 Boiler Fault Code
Gas group fault
Reason(s):
- Combustion quality outside the permitted range
- Wrong gas restrictor
- Recirculation
- Wrong Gas Family
- Blocked internal pressure measuring point
- Gas family check prematurely terminated
F.93 Error Fix
Call out a Gas Safe engineer to check your boiler to determine the cause of the error and fix it.
Vaillant FAQs
How Do I Restart My Vaillant Boiler?
Locate the boiler reset button, commonly found close to the display screen. Note that some boilers have a reset dial as opposed to a button.
Make sure you have resolved any issues that may have caused the lockdown.
Once all issues have been fixed, reset your boiler by following the steps below:
- Press and hold the reset button for up to 10 seconds
- Check to see if the flame or pilot light has ignited
- Make sure any error codes have disappeared
- Check the boiler is working by testing the hot water and heating
Can I Fix a Boiler Myself?
For safety reasons, all errors should be looked at by a Gas Safe Registered engineer.
This is to ensure that any work on your gas appliances is being carried out to the correct safety standard under the Gas Safety Regulations 1998, keeping you, your family, and your home safe.
I Keep Getting Error Codes, Should I Replace My Boiler?
If your Vaillant boiler is fairly old, keeps displaying error codes, and is failing to work properly, it may be time to replace it with a new boiler.
At Boilers Direct, we offer a wide range of high-quality, reliable boilers with installation from a Gas Safe engineer.
Simply answer a few questions about your home and existing boiler and we’ll give you fixed prices on brand new boilers suitable for your home.