Return Thermistor Fault High

Keston Return Thermistor Fault Fault Code

Also written as RETURNTHERMISTORFAULT — same fault.

The Keston Return Thermistor Fault fault code means: your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing it from heating correctly.. It appears on 5 Keston models in our database.

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.

Technical description: Error in the return thermistor circuit located on the pump housing. Checks: thermistor resistance values and wiring continuity to PCB.
Typical repair cost £90-180 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Keston models that show the Return Thermistor Fault fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Keston Return Thermistor Fault fault code information on this page is sourced and cross-checked against the official Keston service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026