High Lockout
U4-66 on Daikin D2CND

Your boiler's internal components have stopped communicating with each other, meaning the system cannot send or receive the signals needed to run.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

CAN-bus timeout

What causes this fault?

This fault occurs when the electrical connection between the boiler's internal computer and its control panel is broken or interrupted. The most common reason is a loose or damaged wiring bridge, but it can also be caused by moisture ingress or a component failure on the circuit board itself. Because these parts can no longer 'talk' to each other, the boiler shuts down as a safety precaution.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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