Indoor unit operates briefly then stops; remote shows communication error; some Daikin systems display U4 on wired controller

Safety first: Disconnect power at the breaker before touching any wiring or opening covers.

Steps to check:

• Locate the communication cable between indoor and outdoor units (often labeled S1/S2/S3 or 1/2/3 on Daikin split systems).
• Verify that the same terminal numbers are connected between indoor and outdoor units (e.g., S1 to S1, S2 to S2, S3 to S3). Correct any miswiring.
• Check that all terminal screws are tight and that no copper conductor is exposed or touching adjacent terminals.
• Inspect the cable for cuts, rodent damage, or crushed sections along its route.

Advanced/technician actions:

• Measure continuity of each communication core from indoor to outdoor; repair or replace the cable if open or shorted.
• Check for correct polarity on any 2-wire communication bus (A/B) if used; reverse if incorrectly wired.
• If wiring is sound, test by substituting a known-good indoor or outdoor PCB (as per Daikin service manual) to identify a faulty board.