Vaillant Boiler Error Codes

Every now and then, your boiler may stop working due to either small or large problems that need to be fixed.

Fortunately, most Vaillant boilers can tell you exactly what is wrong with an error code, so you know whether or not it’s just a quick fix or if you need to call out a boiler engineer.

In this guide, we’ll be taking a look at some of the most common Vaillant boiler error codes, what they mean and how you can fix them. 

Please note: Apart from minor fixes such as defrosting your condensate pipes, bleeding your radiators or topping up your boiler pressure, 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.

Common Vaillant Boiler Error Codes, What They Mean and How to Fix Them

Vaillant F.00 & F.01 Error Fault Codes

Error Description

Interruption to Heating Flow Temperature Sensor or Heating Flow NTC Thermistor fault.

Error Cause

• Unplugged NTC plug

• Loose NTC plug

• Multiple plugs on the PCB not plugged in correctly

• Interruption in cable harness

• Defective NTC

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.02 Boiler Error Code

Error Description

Interruption to DHW outlet sensor in conjunction with F.91 error.

Error Cause

• Defective NTC

• Defective NTC cable

• Defective NTC plug connection

• Defective actoSTOR electronics plug connections

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.03 Boiler Error Code

Error Description

Interruption to the cylinder sensor actoSTOR (NTC) in conjunction with F.91.

Error Cause

• Defective NTC

• Defective NTC cable

• Defective NTC plug connection

• Defective actoSTOR electronics plug connections

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.10 Boiler Error Code

Error Description

Short circuit in Flow NTC fault.

Error Cause

• Defective NTC

• Short circuit in cable harness, cable or casing

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.11 Boiler Error Code

Error Description

Heating Flow NTC Thermistor Fault or Heating Return NTC Short Circuit

Error Cause

• Short circuit on the Heating Return NTC Thermistor (should read 12Kohms @ 20C)

• Wiring harness fault

• Heating Return NTC Thermistor and Main PCB connection error

Error Fix

Gas Safe engineer 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

Error Description

Short circuit in cylinder charging sensor (NTC) in combination with F.91.

Error Cause

• Mass short in sensor plug to casing

• Short circuit in wiring loom

• Fault sensor

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

Error Description

• VU-/VUW boiler: Short circuit warm start sensor/cylinder sensor

• VUW boiler with actoSTOR: Short circuit cylinder sensor (NTC)

In combination with F.91/

Error Cause

• Defective NTC

• Short circuit in cable harness, cable or casing

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

Error Description

Safety Switch-Off: Temperature Limiter.

Error Cause

Boiler has overheated and must cool down.

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

Error Description

Safety Switch-off: low water pressure.

Error Cause

• No water pressure

• Pressure too low in boiler

• Defective water pressure sensor

• Loose, Disconnected or defective cable to pump

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 external filling loop for system boilers.

Vaillant F.23 Boiler Fault Code

Error Description

Safety switch-off: Temperature difference too great.

Error Cause

• Inadequate pump operation

• Blockage or air lock within the appliance

• Blockage or air lock within the heating system

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.24 Boiler Fault Code

Error Description

Water circulation fault (temperature rise greater than 10ºC per second).

Error Cause

• Inadequate pump operation

• Isolation valves closed under appliance

• Blockage or air lock within the appliance

• Blockage or air lock within the heating system

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

Error Description

Safety shut-off: flue gas temperature is too high.

Error Cause

• Flue gas temperature limiter STB is faulty

• Flue gas temperature limiter STB is sooted up

• Broken plug

• Fault cable harness

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

Error Description

• Safety switch-off: Flame simulation

• Incorrect flame detection fault

Error Cause

• Damaged electrode or ignition lead

• Moisture on electronics

• Spark generator fault

• Main PCB fault

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

Error Description

Failure during start-up: unsuccessful ignition.

Error Cause

• 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)

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

Error Description

Operation failure: re-ignition unsuccessful.

Error Cause

• Temporary stop in gas supply

• Flue gas recirculation

• Blocked condensate duct

• Defective earthing of product

• Spark failure in ignition transformer

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

Error Description

Fan Fault.

Error Cause

• Fan plug incorrectly plugged in

• Multiple plug on PCB incorrectly plugged in

• Broken cable harness

• Blocked fan

• Defective hall sensor

• Defective electronics

Error Fix

Call out a Gas Safe engineer to investigate and fix the problem.

Vaillant F.35 Boiler Fault Code

Error Description

Air/flue gas duct fault.

Error Cause

Blocked flue.

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

Error Description

Short circuit or interruption in coding resistor output range located in the cable harness or gas group resistor.

Error Cause

Issue between the coding resistor and PCB, sometimes in combination with (or as a result of) F.70.

Error Fix

Call out a Gas Safe engineer to check the connections between the coding resistor and PCB. 

Vaillant F.49 Boiler Fault Code

Error Description

eBUS fault.

Error Cause

• eBUS short circuit

• eBUS overload

• Two power supplies have different polarities on the eBUS

Error Fix

This issue requires a Gas Safe engineer.

Vaillant F.52 and F.53 Boiler Fault Codes

Error Description

There is an issue with the mass flow sensor.

Error Cause

• Sensor connection fault

• Faulty flow sensor function

• Wet filter cap

• Blocked filter cap

• Gas flow pressure too low

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

Error Description

Fault in the gas supply. Often accompanied by an F.28 or F.29 error code.

Error Cause

• Insufficient gas supply

• Closed gas isolator on boiler or elsewhere

• Gas flow pressure too low

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

Error Description

Mass flow sensor regulation fault.

Error Cause

• Defective gas valve

• Defective cable harness to gas valve

Error Fix

Call out a Gas Safe engineer to test the gas valve and replace it if necessary.

Vaillant F.57 Boiler Fault Code

Error Description

Fault during comfort safety mode.

Error Cause

Highly corroded ignition electrode.

Error Fix

Call out a Gas Safe engineer to inspect the ignition electrode and replace it as necessary.

Vaillant F.61 Boiler Fault Code

Error Description

Gas valve actuation fault.

Error Cause

• Defective cable harness supply line to gas valve

• Defective gas valve

• PCB defective

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

Error Description

Gas valve switch off delay.

Error Cause

• Delayed shutdown of gas valve

• Defective gas valve

• Defective PCB

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 replacing.

Vaillant F.63 Boiler Fault Code

Error Description

Problem with the circuit board (PCB).

Error Cause

• Old age - general wear and tear

• Loose or damaged connections to and from the PCB

• Incorrectly setup PCB following installation or service

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

Error Description

Issue with the electrics: 

• Faulty NTC connections

• Fault PCB

Error Cause

• Short-circuit in flow

• Return NTC

• Defective electronics

Error Fix

Call out a Gas Safe engineer to test your electronics and fix the issue.

Vaillant F.65 Boiler Fault Code

Error Description

Main PBC temperature is too high.

Error Cause

• Problem with the mains input voltage

• Loose or damaged electrical connection on main PCB

• Main PCB fault

Error Fix

Call out a Gas Safe engineer to test your electronics and fix the issue.

Vaillant F.67 Boiler Fault Code

Error Description

Flame Signal Fault on Main PCB.

Error Cause

• Loose or damages electrical connection on main PCB

• Faulty Main PCB

• Problem with the Mains Input Voltage (reverse polarity, poor earth etc)

Error Fix

Call out a Gas Safe engineer to test your electronics and fix the issue.

Vaillant F.68 Boiler Fault Code

Error Description

Unstable flame signal.

Error Cause

• Insufficient inlet working gas pressure to 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

Error Fix

Due to the high number of possible causes for this error code, its best to call out a Gas Safe engineer to test your electronics and fix the issue

Vaillant F.70 Boiler Fault Code

Error Description

Invalid device specific number (DSN).

Error Cause

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.

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 DNS can be assigned through another interface.\

Vaillant F.71 Boiler Fault Code

Error Description

Disconnected or defective heating flow NTC thermistor.

Error Cause

• Heating flow NTC thermistor isn’t connected to flow pipe on main heat exchanger correctly

• Defective heating flow NTC thermistor

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

Error Description

Permanent temperature difference between the flow and return NTC thermistors.

Error Cause

• Incorrect resistance on the flow and return thermistors

• Wiring issue between the NTC thermistor and the PCB

• Thermistors have been fitted ot incorrect pipes during installation

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. This costs between £10-£30 for the part and additional labour costs.

Vaillant F.73 and F.74 Boiler Fault Codes

Error Description

The water pressure sensor is too low.

Error Cause

• Loose, damaged or broken connection between the sensor and PCB

• Sensor and main PCB connection has water damage

• Faulty pressure sensor

Error Fix

Call out a Gas Safe engineer to check for issues with the wiring and water pressure sensor. If a new sensor is needed, it will likely cost £50 plus any additional labour costs.

Vaillant F.75 Boiler Fault Code

Error Description

No pressure change detection when starting-pump.

Error Cause

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.

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

Error Description

Overheating protection on the primary heat exchanger has been triggered.

Error Cause

• Fused cable or cable connections for safety in primary heat exchanger

• Defective primary heat exchanger

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 replacing the boiler entirely.

Vaillant F.77 Boiler Fault Code

Error Description

Fault with the flue non-return flap or condensate pump.

Error Cause

Condensate pump blockage or malfunction.

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

Error Description

Flow and/or return temperature sensor fault.

Error Cause

• Temperature change registered at flow an/or return sensor is non existent or too small

• Not enough water in boiler

• Flor and/or return temperature sensor not in correct position at pipe

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

Error Description

Permanent temperature difference on the NTC flow/return thermistors.

Error Cause

• Flow and return temperature sensors return implausible values

• Flow and return temperature sensors are inverted

• Flow and return temperature sensors are incorrectly fitted

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

Error Description

Flow and return temperature sensors incorrectly fitted.

Error Cause

Flow and return temperature sensors have been fitted to the same pipe or wrong pipe.

Error Fix

Call out a Gas Safe engineer to manually check reading for both temperature and resistance and either reinstall or replace the thermistors. Replacements cost around £150-£150.

Vaillant F.92 Boiler Fault Code

Error Description

Coding resistor fault.

Error Cause

The coding resistor on the PCB doesn’t match the entered gas family.

Error Fix

Call out a Gas Safe engineer to check the resistor and enter the correct gas family.

Vaillant F.93 Boiler Fault Code

Error Description

Gas group fault.

Error Cause

• Combustion quality outside the permitted range

• Wrong gas restrictor

• Recirculation

• Wrong Gas Family

• Blocked internal pressure measuring point

• Gas family check prematurely terminated

Error Fix

Call out a Gas Safe engineer to check your boiler to determine the cause of the error and fix it.

Vaillant Error Codes FAQs

How Do I Restart My Vaillant Boiler?

To reset your Vaillant boiler, you’ll need to locate the boiler reset button. This is most commonly found close to the display screen, however some boilers have a reset dial as opposed to a button.

Prior to resetting your boiler, 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 My Boiler Myself?

Apart from minor fixes such as defrosting your condensate pipes, bleeding your radiators or topping up your boiler pressure, 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 iHeat, 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.

Other Boiler Error Codes

Have you got a Viessmann, Worcester Bosch, Alpha or Ideal boiler with an error code?

Check out our other error code guides here:

• Common Alpha Error Codes

• Common Ideal Error Codes

• Common Viessmann Error Codes

• Common Worcester Bosch Error Codes

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