BoilerFaultCodes Fault 03 / 21 Medium Lockout
Your boiler's built-in thermostat controls has developed an electrical fault and can no longer tell how warm your room is.
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: Room temp sensor faulty: There is a short circuit on the room temperature sensor built into the FW 100.
What causes this fault?
This fault occurs when the electrical components inside the thermostat unit fail, preventing it from sending a clear signal to the boiler. The most common reason is an internal short circuit or a degraded sensor probe that can no longer accurately measure the air temperature in your home.
Typical repair cost £120-220 (including labour)
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 Fault 03 / 21 — no call-out fees, no surprises.
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