BoilerFaultCodesYour boiler has detected an issue with the system that removes waste water or measured a temperature imbalance, causing it to shut down as a safety precaution.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Condensate pump / flue gas thermistor fault
What causes this fault?
This fault occurs when the boiler detects a failure in the pump responsible for removing waste water (condensate) or a safety sensor indicates the internal temperatures are incorrect. The most common reason is a blocked or failed condensate pump, which prevents the boiler from safely draining liquid produced during operation. If the water cannot escape, or if a sensor detects abnormal heat levels, the system shuts down to prevent internal damage or flooding.
This fault is more frequent during winter, as the external drain pipe can freeze, causing water to back up and overwhelm the condensate pump.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like F77 — no call-out fees, no surprises.
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