BoilerFaultCodesYour boiler is still providing heat and hot water, but it has lost communication with your smart room thermostat, meaning you cannot control the temperature remotely.
This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.
Technical description: OpenTherm room thermostat connection is defective – your boiler will continue to operation but the thermostat is out of operation
What causes this fault?
This fault occurs when the digital conversation between your boiler and your smart thermostat is interrupted, usually due to a loose wire or a failing electronic component in the control box. The most common reason is a break in the physical communication cable or a software glitch in the thermostat receiver that prevents it from sending instructions to the boiler.
DIY fix possible
Check that your room thermostat has power and functional batteries.Ensure the thermostat bridge or receiver is plugged in and switched on.Inspect the wiring connection between the thermostat wall plate and the boiler for any loose cables.
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 U4-65 — no call-out fees, no surprises.
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