BoilerFaultCodesYour boiler's internal thermometer is broken, meaning it cannot accurately tell how hot the water is and has stopped working 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: Resistance does not increase from approx 2.5K to 10K as boiler cools; replace.
What causes this fault?
This fault occurs when the internal sensor responsible for monitoring water temperature fails due to electrical degradation or internal corrosion. Over time, the sensitive components inside the thermistor can wear out or become coated in limescale, preventing the sensor from sending the correct electrical signal to the boiler's main control board.
This issue often becomes more apparent during autumn when the heating is turned on for the first time after summer, as the sensor struggles to cope with the sudden increase in thermal stress.
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.
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A boiler cover plan would cover faults like Faulty CH thermistor — no call-out fees, no surprises.
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