boilers
Worcester Bosch Boiler Fault Codes: Complete Guide for London Homes
Worcester Bosch Boiler Fault Codes
Worcester Bosch is the UK's most popular boiler brand, found in millions of London homes. Their Greenstar range displays letter and number fault codes on the controller. Here is what each code means.
EA338 — Ignition Lockout
EA338 is Worcester Bosch's ignition failure code — the boiler attempted to light the burner but failed. Common causes: dirty burner, gas valve fault, or blocked flue.
Action: Reset once. If repeated, Gas Safe engineer required. Do not reset more than twice — repeated failed ignitions can allow gas to accumulate.
E9 — Overheat Thermostat
E9 indicates the boiler has overheated and the safety thermostat has tripped. Often caused by a failed pump (water not circulating) or blocked heat exchanger. Kettling is common in London due to hard water limescale.
E5 — High Flow Temperature
E5 means the flow temperature has exceeded safe limits. Often indicates a circulation problem — blocked system filter, failed pump, or sludge restricting flow.
A1 — Fan Fault
A1 means the boiler fan is not reaching the required speed. The fan creates draught to exhaust combustion gases safely. A fault here is a safety issue — call a Gas Safe engineer promptly.
CE 207 — Temperature Sensor
CE 207 indicates a temperature sensor (NTC thermistor) fault. The boiler cannot accurately measure temperature so locks out for safety.
Frequently asked questions
How do I reset a Worcester Bosch boiler?
Press and hold the reset button (marked with a flame and cross symbol) for 3 seconds. The boiler will attempt to restart. If the same fault code reappears, do not continue resetting — the underlying fault needs professional diagnosis.
Why is my Worcester Bosch boiler kettling?
Kettling — a banging or rumbling noise from the boiler — is caused by limescale on the heat exchanger. London is a hard water area, making kettling particularly common. A power flush combined with a limescale inhibitor injection prevents recurrence.
Does annual service prevent Worcester Bosch faults?
Yes — annual service catches developing faults early, keeps the burner clean (preventing EA338), checks the pump and pressure, and tops up inhibitor. Worcester Bosch requires annual service to maintain the manufacturer warranty.