Return Thermistor High

Baxi Return Thermistor Fault Code

Also written as RETURNTHERMISTOR — same fault.

The Baxi Return Thermistor fault code means: your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, causing it to stop running as a safety precaution.. It appears on 3 Baxi 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: LED 1: Flash, LED 2: On, LED 3: On, LED 4: Flash (Go to Return Thermistor section of the fault finding instructions)
Typical repair cost £90-160 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Baxi models that show the Return Thermistor fault code

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

Source

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

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