High
F8 on Glow-worm Xtramax HE

Your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing the system from heating correctly.

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

Central Heating return thermistor fault.

From this model's manual
Verified against the Glow Worm Xtramax He installation & service manual, page 33.
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 sensor responsible for monitoring the temperature of the water returning to the boiler sends an incorrect signal or fails entirely. It is usually caused by the internal components of the sensor wearing out over time, or by damaged wiring that prevents the boiler's computer from receiving accurate data.

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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