Short answer
The flow sensor tells the boiler how hot the water leaving it is. Most are NTC thermistors whose resistance falls as temperature rises. An open circuit, short circuit or drifting sensor gives the boiler wrong information, causing lockouts, overheating or failure to reach temperature.
Common causes
Sensor drift with age.
Poor thermal contact or missing paste on clip-on sensors.
Damaged or corroded wiring.
Water ingress into the sensor.
What the engineer checks
Measure resistance at a known temperature and compare with the manufacturer's table.
Check wiring and connector condition.
Repair and prevention
Replace the sensor, ensure good contact and secure wiring. Check the return and high-limit sensors at the same visit.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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