Furnace displays flow temperature sensor fault, may not start or may run erratically

Safety: Turn off power before accessing the control compartment.

• Locate the flow NTC: On Baxi furnaces/boilers, the flow NTC is usually clipped or screwed into the primary flow pipe near the heat exchanger.
• Check connections: Inspect the NTC plug and wiring harness for corrosion, loose pins, or damage. Reseat the connector firmly.
• Resistance test: With the sensor disconnected and the system cold, measure resistance with a multimeter and compare to Baxi's temperature/resistance chart. An open or very low resistance reading indicates a faulty sensor.
• Replace if faulty: If the NTC is defective, drain down the primary circuit if required, remove the sensor, fit a new Baxi-compatible NTC with thermal paste if specified, and refill/vent the system.

Note: If a known-good sensor still gives an error, the main PCB may be faulty and should be checked by a professional.