FieldBench
Mitsubishi codes

Mitsubishi

U4

Lockout

Communication

Indoor–outdoor transmission.

Walk U4 in Diagnose

3 plates — walk the one on the cabinet

First check

1 Data pair, not 24 V

On Mitsubishi MSZ-FS 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. S1/S2/S3 landing, polarity, no 240 on comm, outdoor PCB LED.

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

The rest of the walkthrough is Diagnose.

On the plates

  • S1/S2/S3 landing, polarity, no 240 on comm, outdoor PCB LED.
  • On an MXZ pair, U4 may actually live on the outdoor / branch-box chart — isolate this head.
  • S1/S2/S3 landing, polarity, no 240 on comm.
  • On MXZ, isolate this indoor — one bad GL will U4 the outdoor.
  • S1/S2/S3 (or box terminals) landing and polarity. No 240 on comm. Never megger the pair.
  • Isolate: disconnect indoors one at a time (power down first). One bad indoor or one bad run will U4 the outdoor.
  • If a branch box is present, U4 after all heads are isolated is box-to-outdoor or the outdoor PCB.
  • Do not assume Daikin pinout.

Plates that use U4

Same number, other makers

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