Vaillant Boiler Error Codes Explained

If your Vaillant boiler is showing an error code on the display, don't panic — it's designed to tell you (and your engineer) what's gone wrong. Some codes point to minor issues you might be able to resolve yourself, while others mean the boiler has locked out for safety reasons and needs a Gas Safe registered engineer.

As a Vaillant Accredited Installer with over 21 years in the trade, I work on Vaillant boilers daily across Surrey. Here's a plain-English guide to the most common error codes, what causes them, and what to do.

The most common Vaillant error codes

F28 — Ignition failure

This is the error code I see more than any other. The boiler has tried to ignite and failed.

Common causes: No gas supply (check your gas meter and prepayment meter if you have one), a faulty ignition electrode, a faulty gas valve, frozen condensate pipe in cold weather, or low gas pressure.

What you can try: Check that your gas supply is on — try another gas appliance like a hob. If it's below freezing, check the condensate pipe (the plastic overflow pipe that goes outside) — pour warm water over it to thaw it. Press the reset button on the boiler once. If the boiler fires up, the issue may have been temporary.

When to call an engineer: If resetting doesn't work, or the boiler keeps showing F28 repeatedly, you need a qualified engineer. The ignition electrode, gas valve, or gas supply may need attention — all of which require Gas Safe registration to work on.

F22 — Low water pressure

The boiler has detected that the system pressure has dropped too low to operate safely.

Common causes: A leak somewhere in the heating system (radiator valves, pipework joints, or the boiler itself), a recently bled radiator that lost pressure, or a faulty pressure sensor.

What you can try: Check the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar. If it's below 1.0, you can top it up using the filling loop (usually a braided metal hose underneath the boiler with a small tap). Open the tap slowly until the gauge reads about 1.2 bar, then close it. Reset the boiler.

When to call an engineer: If the pressure keeps dropping after you've topped it up, there's a leak in the system that needs finding and fixing. Repeatedly topping up the pressure isn't a solution — it's adding fresh water which causes more corrosion and sludge buildup.

F75 — Pump fault

The boiler can't detect that the pump is running, so it shuts down to prevent overheating.

Common causes: A seized pump (common after summer when the heating hasn't been used for months), air in the pump, a faulty pump, or a wiring issue.

What you can try: Unfortunately, not much. This one almost always needs an engineer.

When to call an engineer: Immediately. A pump replacement is a common repair and usually straightforward — typically £250–£400 including parts and labour, depending on the model. Sometimes the pump can be freed without replacing it if it's simply seized from lack of use.

F29 — Flame loss during operation

The boiler was running fine but the flame went out unexpectedly.

Common causes: Intermittent gas supply issue, a dirty or failing flame sensor, a blocked or partially blocked flue, or high winds affecting the flue terminal.

What you can try: Reset the boiler once. If it fires up and runs normally, it may have been a one-off caused by wind conditions or a brief gas supply fluctuation. If it happens again, call an engineer.

When to call an engineer: If F29 keeps recurring. A blocked flue is a safety issue that needs checking urgently. A failing flame sensor is a relatively inexpensive fix.

F20 — Overheat protection

The boiler has overheated and shut down for safety.

Common causes: Air in the system, a circulation problem (pump not working properly, blocked pipes), or a faulty overheat thermostat. In systems with sludge buildup, restricted flow through the heat exchanger can cause localised overheating.

What you can try: Let the boiler cool down for 10 minutes, then try resetting. If it fires up and runs normally, monitor it. If it keeps cutting out with F20, call an engineer.

When to call an engineer: Repeated F20 errors suggest a circulation problem that needs diagnosing. If the system hasn't been power flushed and the boiler is overheating, sludge buildup may be restricting flow through the heat exchanger.

F27 — Flame detection fault

The boiler is detecting a flame when there shouldn't be one, or the flame detection electronics are faulty.

Common causes: A faulty flame sensor, moisture in the boiler electrics, or a wiring issue.

What you can try: This requires an engineer — don't try to reset repeatedly.

When to call an engineer: Promptly. This is a safety-related fault that needs professional diagnosis.

F62 — Gas valve delay fault

The gas valve isn't closing quickly enough after the boiler shuts down.

Common causes: A failing gas valve, a wiring issue, or a faulty PCB (printed circuit board).

What you can try: Nothing — this needs an engineer.

When to call an engineer: This is a safety-critical fault. The gas valve is responsible for stopping gas flow when the boiler shuts down, so any delay in closing is taken seriously.

F83 — Temperature differential fault

The boiler is detecting an abnormal temperature difference between the flow and return pipes.

Common causes: Low water in the system, air locks, a pump running too slowly, or sludge in the heat exchanger restricting water flow.

What you can try: Check the system pressure and top up if needed. Bleed radiators to release any trapped air.

When to call an engineer: If topping up the pressure and bleeding radiators doesn't resolve it, the pump speed or heat exchanger may need attention. In older systems, a power flush often resolves this by clearing the sludge that's causing the restriction.

Quick reference: Vaillant error codes at a glance

When to reset vs when to call an engineer

It's fine to reset your boiler once or twice when you see an error code. The reset button is there for a reason — sometimes a temporary glitch triggers a lockout and a reset clears it. If the boiler fires up and runs normally after a reset, keep an eye on it but don't worry.

Stop resetting and call an engineer if: the same error code keeps coming back, you've reset the boiler more than three times in a day, you can smell gas, or the boiler is making unusual noises. Repeatedly resetting a boiler that keeps locking out on the same fault can make the underlying problem worse.

Never attempt to remove the boiler casing or work on any gas components yourself. All gas work must be carried out by a Gas Safe registered engineer.

How to prevent error codes

Most of the faults that trigger error codes can be prevented with basic maintenance:

Annual servicing. A proper boiler service catches worn components before they fail. The ignition electrodes, flame sensors, and gas valve that cause F28, F27, and F62 are all checked and replaced if needed during a service.

Keep the system clean. Sludge is the underlying cause of many error codes — F20, F75, and F83 can all be traced back to dirty system water restricting flow. A power flush and a magnetic system filter keep the water clean and protect the boiler.

Protect the condensate pipe. If your condensate pipe runs outside and is exposed to the elements, lagging it with pipe insulation prevents the winter freeze-ups that cause F28 lockouts. This is one of the most common callouts in cold weather and one of the easiest to prevent.

Maintain system pressure. Keep an eye on the pressure gauge and top up when it drops below 1.0 bar. If it keeps dropping, get the leak found and fixed rather than just topping up.

Need help with a Vaillant error code?

We repair and service Vaillant boilers across Surrey from our base in Addlestone, covering Weybridge, Chertsey, Walton-on-Thames, Woking, Byfleet, Shepperton, Egham, Staines, Ashford, and Englefield Green.

As a Vaillant Accredited Installer, I carry Vaillant parts in the van and have direct access to Vaillant's technical support line. Most Vaillant faults can be diagnosed and repaired in a single visit.

Call me on 07311 811574 or get in touch — I can often help you work out what's wrong over the phone before we even book a visit.