F.1 on a Vaillant boiler means: return temperature sensor interrupted. One of the boiler's sensors is giving a reading the control board can't trust, so it has stopped to be safe.
This code applies to Vaillant ecoTEC. Codes can mean different things on different models, so check the display against your boiler's user manual.
What you can safely check
- Look at the pressure gauge on the front of the boiler. With the heating off and the system cold, it should normally read 1 to 1.5 bar.
- Turn the boiler off at the switch or fused spur, wait 30 seconds, and turn it back on. Then reset once if needed.
When to call an engineer
If the code stays or keeps coming back, the sensor, its wiring or connector usually needs replacing. Sensors are generally an inexpensive part, but they are inside the casing, so this is a job for a Gas Safe engineer.
Never remove the boiler casing yourself — everything behind it must be handled by a Gas Safe registered engineer. If you smell gas, open windows, turn the gas off at the meter, and call the National Gas Emergency number on 0800 111 999.
Repair it, or replace it?
If your boiler is over 10 years old, compare a repair against a new combi before you pay for parts.
Applies to: Vaillant ecoTEC
Meaning checked against: Vaillant — boiler fault codes
Last reviewed November 2026. Always follow your boiler's own user manual.