BoilerFaultCodes 12 Medium Warning
Your boiler has lost connection with the sensor that monitors the temperature of your hot water cylinder, meaning it may not know when to heat your water.
Book a service
This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.
Technical description: Cylinder sensor fault (FlowSmart only)
What causes this fault?
This fault occurs when the main control board can no longer detect an electrical signal from the temperature probe attached to your hot water tank. It is usually caused by the sensor internal components failing over time or a loose connection in the wiring that links the cylinder to the boiler.
Typical repair cost £90-160 (including labour)
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 12 — no call-out fees, no surprises.
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