U4-66 High

Daikin U4-66 Fault Code

Also written as U466 — same fault.

The Daikin U4-66 fault code means: your boiler's internal components have stopped communicating with each other, meaning the system cannot send or receive the signals needed to run.. It appears on 1 Daikin model in our database.

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 the Daikin U4-66 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.

Typical repair cost £150-£450 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Daikin models that show the U4-66 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Daikin U4-66 fault code information on this page is sourced and cross-checked against the official Daikin service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026