FieldBench
Goodman codes

Goodman

U / Comm

Lockout

ComfortBridge / U-style comm

Indoor-outdoor data pair. Valid only on a communicating pair (GMVC / AVPTC / AMVC). On a 24V furnace indoor this is a miswire, not a failed outdoor board.

Walk U / Comm in Diagnose

Sign in for the next measurement

First check

1 Data pair, not 24 V

On Goodman GSZC18 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. Data pair polarity, splices, outdoor board power.

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

The rest of the walkthrough is Diagnose.

On the plates

  • Data pair polarity, splices, outdoor board power.
  • Indoor must be a communicating IFC. GR9T96 / GMEC96 / GMSS96 cannot clear a U / comm fault on data wire.
  • U4-class comm with a 24V furnace: land Y1/Y2/O/B/C or change the indoor. The outdoor will not learn 24V through the data pins.

Plates that use U / Comm