BoilerFaultCodesYour boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, causing it to stop working for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: RETURN THERMISTOR FAULT
What causes this fault?
This fault occurs when the sensor responsible for measuring the temperature of the water entering the boiler fails or sends an incorrect signal. The most common cause is the sensor itself becoming faulty or corroded, but it can also be triggered by a loose wire or a communication error within the main control board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F 05 OR L 05 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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