Vaillant Boiler Fault Codes Explained
Vaillant fault codes (F-codes) tell you which part of the boiler has a problem — but they don't tell you how to fix it safely. Here's what each common code means, what you can check yourself, and when to call a Gas Safe engineer.
F.22 Fault
Flow/return sensor or low water — boiler can't confirm system pressure or temperature.
F.28 Fault
Boiler won't light — ignition attempt failed, no flame detected. No heating or hot water.
F.29 Fault
Flame lost during operation — boiler lit then cut out. Often gas supply or flame sensor.
F.75 Fault
No pressure change when pump starts — pump or pressure sensor, no circulation detected.
F.73 Fault
Pressure sensor or low-pressure problem — boiler can't confirm there's enough water.
When to call a Gas Safe engineer
Any work inside the boiler casing — replacing sensors, the igniter, flame electrode, gas valve, pump or PCB — is gas work that must be done by a Gas Safe Registered engineer. The safe checks on each fault-code page are limited to what you can do without opening the boiler.
If you can smell gas, turn off the gas at the meter, open windows, don't use switches, and call the National Gas Emergency Service on 0800 111 999 immediately.