High
LED1 flashing, LED2 on on Ideal Mini S18

Your boiler has detected a problem with the sensor that monitors the temperature of your central heating water, meaning it can't heat your radiators safely.

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

Faulty CH thermistor or wiring

From this model's manual
Verified against the Mini He 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.

What causes this fault?

This fault occurs when the internal sensor responsible for tracking the temperature of your radiators fails to send a reliable signal to the boiler's control board. The most common reason is that the sensor has physically degraded over time or the electrical connection has become loose, preventing the boiler from knowing when to safely turn the burner on or off.

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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