High
343 on Worcester Bosch Greenstar 12i System

Your boiler has detected that the exhaust gases are becoming dangerously hot while the water it is trying to heat remains cold, causing it to shut down to prevent damage.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

ED CH: flue gas temperature rising but supply temperature not

From this model's manual
Verified against the Worcester Bosch Greenstar 12i System installation & service manual, page 64.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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