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Last updated: 13th June, 2024
GuidesRead all about the most common Vaillant boiler error codes, what they mean and how you can fix them.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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
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.\
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
Effectively, new boiler cost can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer.
Here at iHeat, we remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.
New boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and boiler installation type. Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (guide only).
Installation Type | Price (inc VAT) | |
Combi to combi swap | £1,845 | |
System to combi conversion | £2,499 | |
New boiler install | £2,899 | |
Back boiler to a combi | £3,299 | |
System to system | £1,945 |
Have you got a Viessmann, Worcester Bosch, Alpha or Ideal boiler with an error code?
Check out our other error code guides here:
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Clearing faults on a Vaillant boiler typically involves resetting the system or addressing specific error codes as per the manufacturer's instructions. For minor issues, a reset can be effective, but persistent faults may require professional troubleshooting.
A communication fault in a Vaillant boiler often indicates issues with the boiler's connection to its controls or other system components. This could be due to wiring problems or control unit malfunctions.
Boiler fault codes are alphanumeric or numeric codes displayed on a boiler's control panel, providing a diagnostic tool to identify specific issues within the boiler system. These codes help in troubleshooting and fixing problems.
If your Vaillant boiler is flashing "00," it typically indicates that the boiler is in standby mode and not currently firing. This may occur when there's no demand for heat.
To fix the "00" code on your boiler, ensure there's a demand for heating or hot water. Check thermostat settings and external controls to ensure they're calling for heat. If the issue persists, consult a professional.
The "00" code on a boiler usually signifies that the boiler is in standby mode, waiting for a signal to start heating. It's a normal status code when no heating is required.
If your boiler displays a fault code or shows signs of malfunctioning, it's generally advisable to turn it off and contact a certified technician for a safety inspection and repair.
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