L4 or F4 High
Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.
Technical description: Control / no flow thermistor fault
Typical repair cost: £100-£220 (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 L4 or F4 — no call-out fees, no surprises.
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Parts commonly replaced
- Flow thermistor
- Return thermistor
- Main PCB
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