Vaillant fault codes explained: meaning, resets & next steps

A Vaillant fault code is your boiler’s way of signalling that something needs attention. It may point to a simple issue, such as low system pressure, but it can also indicate an ignition, pump or sensor fault that requires a qualified professional.

This guide explains where codes appear, what common Vaillant faults can mean and which steps are safe for homeowners. Treat it as a clear starting point, not a substitute for a Gas Safe engineer or the instructions for your particular model.

Where Vaillant fault codes appear

On popular Vaillant ecoTEC boilers, a fault code normally appears on the front display, often alongside a warning symbol or a message such as “F.22” or “F.28”. Older models may show the code through an LCD screen or status indicators, so check your boiler manual if the display is unclear.

Before attempting anything, write down the exact code and any accompanying message. Check whether your heating controls are switched on, make sure the programmer or thermostat is calling for heat, and look at the boiler pressure gauge if your system has one.

• Many sealed systems show a normal cold pressure of roughly 1.0 to 1.5 bar, although the correct range varies by model.

• Do not remove the boiler casing or interfere with gas, electrical or combustion components.

• If the condensate pipe is frozen, follow safe guidance in our frozen condensate pipe guide before trying a reset.

• Use the manufacturer’s stated reset process and avoid repeatedly resetting the appliance.

A reset can clear a temporary interruption, but it does not repair the underlying problem. If the same code returns, or the boiler locks out again, stop resetting it and arrange a professional inspection.

Common Vaillant fault codes explained

The following quick-reference guide covers commonly searched Vaillant codes. Exact meanings can differ slightly between models, so always compare the code with Vaillant’s documentation for your appliance.

F22: Low system pressure

What it indicates: The boiler has detected insufficient water pressure in the heating system. Likely causes: water loss from a radiator, valve or pipework, air bleeding from radiators, or a system that needs repressurising.

Safe checks: Read the pressure gauge and look for obvious leaks around accessible radiators and valves. Do not keep adding water if the pressure repeatedly falls. Gas Safe job: Repressurising may be permitted by the manufacturer’s instructions, but recurring low pressure needs a qualified heating engineer to find the cause.

F28: Ignition unsuccessful

What it indicates: The boiler has failed to ignite during an attempted start. Likely causes: interrupted gas supply, ignition or flame-sensing problems, a frozen condensate pipe, or a fault with the gas valve or control system.

Safe checks: Confirm that other gas appliances work, if safe to do so, and check whether the condensate pipe is visibly frozen. You can read our dedicated Vaillant F28 fault code guide for a deeper explanation. Gas Safe job: Any investigation involving the gas supply, ignition, burner or internal boiler components must be completed by a Gas Safe registered engineer.

F75: Pressure sensor or pump fault

What it indicates: The boiler has not detected the expected pressure change when the pump starts. Likely causes: a faulty pressure sensor, air in the system, restricted circulation, low system pressure or a pump problem.

Safe checks: Check the displayed pressure and make sure the boiler has not simply lost pressure. Do not attempt to access or replace the pump or sensor. Gas Safe job: Diagnosis and repair normally require an engineer, particularly if the code returns after one manufacturer-approved reset.

Safety checks before calling for help

Never ignore signs of a gas or carbon monoxide emergency. If you smell gas, do not operate electrical switches, use naked flames or try to locate the leak. Move outside and call the National Gas Emergency Service on 0800 111 999.

Carbon monoxide has no smell and can be produced by faulty or poorly maintained appliances. Fit a suitable audible carbon monoxide alarm in the same room as the boiler, following the manufacturer’s positioning guidance, and arrange regular boiler servicing. If anyone experiences headaches, dizziness or nausea that improve outdoors, leave the property and seek urgent advice.

Resetting a Vaillant boiler safely

Use only the reset button or control sequence described in your Vaillant manual. Keep resets to a minimum, as repeated attempts can mask a developing fault and will not resolve issues with gas supply, circulation or damaged components.

If the code clears and the boiler operates normally, monitor it and make a note of what happened. If the code returns, the boiler makes unusual noises, pressure drops again or it will not provide heating or hot water, switch it off if appropriate and book a qualified repair.