BoilerFaultCodes 5B:0 Medium Lockout
Your boiler has detected that the hot water cylinder is connected directly to the unit, but there is likely a communication error or a faulty sensor preventing it from heating your water correctly.
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 cylinder directly connected to the boiler
What causes this fault?
This fault occurs when the boiler loses its connection to the temperature sensor inside your hot water tank. The most common reason is a faulty sensor or a loose wire in the cable harness, meaning the boiler can no longer tell how hot your water is and stops heating it as a safety precaution.
Typical repair cost £120-250 (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.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like 5B:0 — no call-out fees, no surprises.
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