Return Thermistor Fault High

Ideal Return Thermistor Fault Fault Code

Also written as RETURNTHERMISTORFAULT — same fault.

The Ideal Return Thermistor Fault fault code means: your boiler is struggling to monitor the temperature of the water returning from your radiators, which prevents it from heating your home or water efficiently.. It appears on 5 Ideal 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: Thermistor resistance value incorrect (9.7-10.3 kΩ at 25°C) or lack of continuity between the PCB and the thermistor.

What causes the Ideal Return Thermistor Fault fault?

This fault occurs when the sensor responsible for measuring the temperature of the water returning to the boiler begins to provide inaccurate readings or fails entirely. The most common reason is the internal electrical component within the sensor wearing out over time, though loose wiring or moisture on the connections can also disrupt the signal sent to the main control board.

Typical repair cost £110-180 (including labour)

Engineer required

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

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Ideal models that show the Return Thermistor Fault fault code

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

Source

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

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026