BoilerFaultCodesYour boiler has lost communication with the sensor that monitors the temperature of your hot water cylinder, meaning it cannot heat your water correctly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Cylinder thermistor fault
What causes this fault?
This fault occurs when the boiler can no longer detect a signal from the sensor that monitors the temperature of your hot water tank. The most common reasons are a breakdown of the internal sensor components or a loose connection in the wiring that links the sensor to the boiler's 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 F12 — no call-out fees, no surprises.
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