High
Red LED on (after safety stat) on Sime Format B Combi

Your boiler has shut down because it has detected an internal temperature that is too high or a problem with how exhaust gases are leaving the flue.

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

100°C safety thermostat (TS) or smoke thermostat (TF) intervention — contact engineer; check for overheating cause

From this model's manual
Verified against the P3 Sime Murelle He 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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