F4 High
Your boiler has detected that a temperature sensor is not communicating correctly, meaning the system cannot tell how hot the water is and has stopped working for safety.
Technical description: Control / No flow thermistor
Typical repair cost: £120-£250 (including labour)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Protect yourself from future repair bills
A boiler cover plan would cover faults like F4 — no call-out fees, no surprises.
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Parts commonly replaced
- Flow Thermistor (NTC Sensor)
- Wiring Harness
- Main PCB
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