Safety: Turn off power and allow the boiler and pipework to cool before touching any components.

Steps:

Locate the flow NTC: On Bosch boilers, the flow NTC is usually clipped or screwed onto the primary flow pipe leaving the heat exchanger.
Check mounting: Ensure the sensor is firmly attached and properly seated with any thermal paste or clip intact. Re‑secure if loose.
Inspect wiring: Follow the cable from the NTC to the PCB. Look for kinks, cuts, or signs of overheating. Reseat the connector at both ends.
Measure resistance: With power off, disconnect the NTC and measure its resistance at room temperature. Compare to Bosch spec. Replace the sensor if readings are open, shorted, or far from expected.
Reset boiler: After reassembly, restore power and reset the boiler. Check that flow temperature readings are stable during operation.

Note: If a new sensor does not clear the fault, the main PCB may be defective and should be checked by a professional.

Boiler displays a flow temperature sensor fault; heating may shut down or run at incorrect temperature.

Possible Causes