BoilerFaultCodesYour boiler has detected that a temperature sensor is disconnected or has failed, which is currently preventing the heating from starting to protect the system from freezing.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Central Heating flow thermistor disconnected (frost protection).
What causes this fault?
This fault occurs when the boiler's control board loses communication with the temperature sensor on the main heating pipe. The most common reason is that the sensor has developed an internal electrical failure or the wiring connecting it to the brain of the boiler has become loose or damaged.
This fault is frequently triggered during the first cold snap of autumn when the system's frost protection kicks in and attempts to read a sensor that has degraded over the summer.
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 F19 — no call-out fees, no surprises.
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