FieldBench
Daikin codes

Daikin

U4

Lockout

Indoor/outdoor comm

Data pair between this outdoor and the communicating indoor is down.

Walk U4 in Diagnose

Sign in for the next measurement

First check

1 Data pair, not 24 V

On Daikin DZ20VC indoor/outdoor comm is a data pair (AB, S1/S2, U, ComfortBridge, ComfortLink — whatever THIS plate uses). It is not a 24 V Y. Check polarity, continuity, and that nobody landed it on R/C. Polarity, gauge, splices on the data pair. This is not 24 V Y.

Expect: Pair continuous, not shorted to chassis, polarity per the diagram.

The rest of the walkthrough is Diagnose.

On the plates

  • Polarity, gauge, splices on the data pair. This is not 24 V Y.
  • Indoor must be communicating (DM96VC / AVPTC / AMVC / GMVC-class). GR9T96 / AR9T96 / 9-speed 24V cannot clear U4.
  • One master. Leftover thermostat cable landed as ‘data’ will never train.

Plates that use U4

Same number, other makers

U4 on Daikin is not the same code on these plates.