Boiler Error Codes: A Guide for Homeowners

What Are Boiler Error Codes?

Modern boilers have onboard diagnostic systems that detect faults and display codes on the control panel. These codes help you — and your engineer — identify the problem quickly. Each brand uses its own coding system, so a code that means one thing on a Worcester Bosch means something different on a Vaillant.

Worcester Bosch Common Codes

E9: Overheat thermostat tripped — the boiler has overheated and shut down for safety. Often caused by a blocked pump or restricted flow.

E1/EA: Ignition lockout — the boiler has tried to ignite and failed. Could be a gas supply issue, spark electrode fault, or gas valve problem.

H7: Heating flow sensor fault — sensor may need replacement.

Vaillant Common Codes

F22: Insufficient water pressure — system pressure is too low. Repressurise the system using the filling loop.

F28/F29: Ignition failure — the boiler is unable to ignite. Check gas supply; may require an engineer for electrode or gas valve inspection.

F75: Pressure sensor or pump fault — the boiler cannot detect a pressure difference when the pump runs.

Ideal Common Codes

F1: Low water pressure — repressurise the system.

F2: Flame failure — ignition issue requiring engineer attendance.

L1/L2: Overheat lockout — call an engineer to investigate the root cause.

What to Do When You See an Error Code

1. Note the code before doing anything else

2. Look it up in your boiler manual (or search online for your model and code)

3. Try the recommended reset procedure if appropriate (usually pressing the reset button for 3 seconds)

4. If the fault returns immediately or repeatedly, call an engineer

Do not keep resetting a boiler that repeatedly locks out — this can mask a potentially dangerous fault.

For error code diagnosis and repair across Surrey, call Halo Heating on 01483 676108.