Vaillant Boiler Fault Codes
Vaillant Boiler Fault Codes Guide
This guide breaks down the most common Vaillant boiler fault codes by category, explaining what each one means, what usually causes it, what you can try first, and when it is time to call a Gas Safe registered engineer. Bookmark this page for the next time your boiler flashes up a code.
Water Pressure and Temperature Faults
What caused it?
System pressure has dropped below the minimum the boiler needs to run safely. This is usually the result of a small leak somewhere in the pipework, a recent radiator bleed that was not topped back up afterwards, or a slow weep from a valve that has gone unnoticed. It is one of the most common Vaillant boiler fault codes homeowners see, and often the easiest to put right.
What to do?
- Locate the filling loop, usually a braided hose or built-in key underneath the boiler.
- Open the valves slowly (or turn the key) and watch the pressure gauge rise.
- Stop once the gauge reaches around 1.0 to 1.5 bar, in the green zone.
- Close the valves fully, or remove the key, then reset the boiler.
Do I call an engineer?
Only if the pressure keeps dropping again within a few days. A boiler that needs repressurising every week or so almost always has a leak that needs finding and fixing properly.
What caused it?
The boiler's high limit thermostat has detected the water temperature climbing above a safe level and has shut the boiler down as a protective measure. This can happen if radiators are blocked and restricting flow, if the pump has slowed down, or if a temperature sensor is giving the control board an inaccurate reading.
What to do?
- Let the boiler cool down fully before touching anything.
- Check that no radiator valves have been accidentally closed.
- Bleed any radiators that feel cold at the top.
- Reset the boiler once and see if it fires normally.
Do I call an engineer?
Yes, if it happens more than once. Repeated overheating points to a pump, sensor or PCB fault that a reset will not solve.
What caused it?
The gap between the flow temperature and the return temperature has become too wide. This normally means water is not circulating properly around the system, which can be caused by a partially blocked pipe, a failing pump, or sludge that has built up inside older radiators and pipework over time.
What to do?
- Check every radiator valve in the property is open.
- Listen for the pump running when the boiler fires up.
- Bleed radiators that are cold at the top or warm at the bottom only.
- Reset the boiler once you have checked the above.
Do I call an engineer?
Yes, this fault typically needs a proper flow check and, on older systems, a power flush to clear built-up sludge.
What caused it?
Water inside the boiler is heating up faster than the control board expects, which usually points to restricted flow around the system or a pump that is not moving water quickly enough to keep up with the burner.
What to do?
- Do not keep resetting the boiler repeatedly.
- Check the pump is running and not making unusual noises.
- Turn the boiler off if you smell anything unusual or hear banging.
Do I call an engineer?
Yes, straight away. This code can indicate a genuine safety issue rather than a simple settings problem.
What caused it?
The heat exchanger inside the boiler is running hotter than it should, most often because it has become dirty, scaled up with limescale, or is struggling to transfer heat efficiently after years of use.
What to do?
- Avoid repeated resets, as this will not clear the underlying build-up.
- Note whether the boiler has had a service in the last twelve months.
- Turn the boiler off if it trips this fault more than once in a day.
Do I call an engineer?
Yes, the heat exchanger will usually need cleaning or descaling by a professional, sometimes alongside a wider service.
Ignition and Flame Faults
What caused it?
The boiler tried to light but did not detect a flame within the expected time. In cold weather this is very often caused by a frozen condensate pipe, the small plastic pipe that carries waste water outside, rather than a genuine fault with the boiler itself. It can also be caused by low gas pressure or a worn ignition electrode.
What to do?
- Find the condensate pipe where it exits the property, usually white plastic.
- Pour warm, never boiling, water over any visibly frozen section.
- Wrap the pipe in lagging or insulation once thawed to prevent it happening again.
- Reset the boiler after thawing and check it fires normally.
Do I call an engineer?
Yes, if the fault returns after thawing the pipe, or if you cannot find a frozen section at all.
What caused it?
The boiler lit successfully but the flame then cut out partway through operation. This can be linked to a poor or interrupted gas supply, a problem with the flue, or a flame sensor that is no longer reading correctly.
What to do?
- Check the gas supply is switched on at the meter.
- Test whether other gas appliances in the property are working.
- Reset the boiler once and note whether the fault returns quickly.
Do I call an engineer?
Yes, if the gas supply is fine elsewhere in the property, this fault needs a professional diagnosis.
What caused it?
The control board believes it can see a flame even though the gas valve is closed. This is almost always a wiring fault or a failing flame sensor rather than anything the homeowner has done, and it is treated as a serious gas safety fault.
What to do?
- Try a single reset in case it was a one-off glitch.
- Do not attempt to inspect or open the boiler casing yourself.
- Turn the boiler off at the isolation switch if the fault repeats.
Do I call an engineer?
Yes, this relates directly to gas safety and should never be left unresolved.
What caused it?
The fan inside the boiler is not reaching, or is not holding, the speed the control board expects it to run at. This can be a wiring issue, a failing capacitor, or the fan itself starting to wear out.
What to do?
- Try one reset from the boiler's control panel.
- Listen for any unusual grinding or rattling from the boiler.
- Avoid repeating the reset if the fault comes straight back.
Do I call an engineer?
Yes, the fan or its wiring will usually need checking and possibly replacing.
Sensor (NTC) Faults
What caused it?
The cable or connection to the flow temperature sensor (F.00) or the return temperature sensor (F.01) has failed, corroded, or simply worked loose over time, so the control board is no longer receiving a reading from it.
What to do?
- Reset the boiler once, as a loose connection can occasionally reseat itself.
- Avoid repeated resets if the fault returns straight away.
- Make a note of exactly which code is showing for the engineer.
Do I call an engineer?
Yes, the sensor or its wiring will need testing with proper equipment and is likely to need replacing.
What caused it?
A short circuit has developed in the wiring for the flow sensor (F.10) or the return sensor (F.11), which can happen through general wear, moisture ingress, or a connector that has come adrift inside the boiler.
What to do?
- Reset the boiler once to see if the fault clears.
- Avoid repeated resets, as this will not clear a genuine wiring fault.
Do I call an engineer?
Yes, this needs proper diagnostic testing to find the source of the short circuit.
Gas Valve and PCB Faults
What caused it?
The cable loom connecting to the gas valve has developed a fault, the valve assembly itself is defective (F.61), or the valve is not closing correctly once the boiler has finished a heating cycle (F.62). Both relate directly to how gas is controlled inside the appliance.
What to do?
- Turn the boiler off at the isolation switch.
- Do not attempt to open the casing or inspect the valve yourself.
- If you smell gas at any point, leave the property and call the National Gas Emergency Service on 0800 111 999.
Do I call an engineer?
Yes, immediately. Both of these are gas safety faults and should be treated as urgent.
What caused it?
This group covers an internal memory error on the control board (F.63), a sensor short circuit or wider electronic fault (F.64), the control board overheating, often due to a nearby heat source (F.65), or a flame detector signal that has moved outside its normal expected range (F.67).
What to do?
- Try a single reset from the control panel.
- Note down the exact code shown, since there are several within this group.
- Avoid repeated resets, as they will not fix a genuine PCB fault.
Do I call an engineer?
Yes, these codes usually point to a faulty printed circuit board that needs professional diagnosis and, in many cases, replacement.
Pump and Communication Faults
What caused it?
The boiler started the pump but did not detect the pressure change it expected to see. This can mean the pump has seized, an air lock has formed in the system, an isolation valve near the boiler has been left closed, or the pressure sensor itself is faulty.
What to do?
- Check that both isolation valves near the boiler are fully open.
- Listen for the pump running when the boiler tries to fire.
- Bleed radiators to help clear any air locks in the system.
- Reset the boiler once you have checked the above.
Do I call an engineer?
Yes, if the valves are open and the fault persists, since a seized pump or faulty sensor will need replacing.
What caused it?
A short circuit, overload, or wiring problem has developed on the eBUS line, the cable that lets the boiler communicate with your room thermostat or smart controls.
What to do?
- Check the thermostat cable connections if you are comfortable doing so.
- Make sure no new controls or accessories have been wired in recently.
- Reset the boiler once you have checked the wiring visually.
Do I call an engineer?
Yes, if a quick visual check does not resolve it, since this often needs proper cable testing.
Trust Vaillant Boiler Experts
Fault codes are a useful starting point, but they only tell part of the story. Knowing which ones are safe to leave overnight and which need immediate attention comes from years of hands-on experience with Vaillant appliances specifically.
Our Gas Safe registered heating engineers work on Vaillant boilers across Essex every week, spotting Vaillant boiler fault codes and fixing what's behind them, from routine servicing to full breakdowns. We use genuine parts wherever possible and explain exactly what we find, so there are no surprises later.
What to Do?
Safe DIY Checklist
- Check boiler pressure is within the normal range, usually around 1.0 to 1.5 bar when cold
- Reset the boiler once to see if the fault clears
- Make sure the gas and electricity supplies are switched on
- Check radiators for cold spots and bleed if necessary
- Ensure the condensate pipe is not frozen or blocked, especially in winter
- Confirm all radiator valves are fully open
- Look for any obvious external obstructions around the flue terminal
Avoid opening the boiler casing or attempting any internal repairs. If a fault code returns after working through this checklist, it is time to call a Gas Safe registered engineer.