High
On / Off on Worcester Bosch Greenstar 12i System

Your boiler is failing to communicate with your thermostat or timer, preventing it from turning on when it should.

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

Boiler not operating during central heating demand (HW ok) OR Boiler operating without live demand at ST10-LR(from external roomstat timer) OR Boiler not operating during hot water demand (CH ok) OR Boiler not operating during any demand.

From this model's manual
Verified against the Worcester Bosch Greenstar 12i System installation & service manual.
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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