BoilerFaultCodes E12 Medium Lockout
Your boiler's hot water sensor has failed or detected an error, meaning the system cannot accurately measure the temperature of your tap 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: DHW sensor S3 defective
What causes this fault?
This fault occurs when the domestic hot water sensor becomes damaged or begins sending incorrect electrical signals back to the main control board. The most common reason is internal wear and tear on the sensor's thermistor, though loose wiring or moisture ingress at the connection point can also trigger the error.
Typical repair cost £100-£180 (including labour)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like E12 — no call-out fees, no surprises.
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