F.78 High
Your boiler has detected a fault with a safety sensor designed to prevent the main heating unit from overheating, causing the system to shut down.
Technical description: The safety cut-out in the primary heat exchanger is defective
Typical repair cost: £140-£280 (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 F.78 — no call-out fees, no surprises.
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Parts commonly replaced
- Overheat thermostat
- Primary Heat Exchanger sensor
- Wiring harness
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