How to Fix / Troubleshooting

Safety: Isolate power and allow boiler to cool.

Steps:

Check connectors: Locate the flow NTC on the flow pipe and ensure its plug is fully inserted. Check the corresponding plug on the PCB.
Visual inspection: Look for any signs of water ingress on the sensor or wiring, especially around compression joints or automatic air vents.
Continuity test: With power off, disconnect both ends of the NTC cable and test continuity. Repair or replace if open or shorted.
Sensor test: Measure NTC resistance and compare to Bosch chart. Replace if faulty.

Note: If wiring and sensor are good, the PCB may need replacement by a qualified engineer.

Boiler shows a flow temperature sensor or sensor circuit fault; heating may not start or may overheat.

Possible Causes