FieldBench

Goodman

GSXC18

AC · Two-stage / communicating outdoor · 18 SEER class

Goodman two-stage communicating condensing unit. Outdoor board talks ComfortBridge when paired with a communicating indoor (GMVC / AVPTC family). Faults live on the OUTDOOR board — not furnace E-codes. Will also run 24V Y1/Y2 against a conventional indoor. Amana ASXC18 is its own plate.

Not GSX14 / GSX16 single-stage piston units (contactor + cap only). Not GSZC18 / GSZC16 heat pump (reversing valve + defrost control). Not ASXC18 (Amana-badge — own chart). Not GLXT7C (R-32 two-stage).

Walk this in Diagnose

This plate

Stages
Two-stage scroll or inverter-adjacent two-step depending on vintage
Metering
TXV indoor required for rating
Pair
ComfortBridge air handler / furnace (data pair) or conventional 24V Y1/Y2
Sister SKU
GSXC16 · Amana ASXC18 / AVXC20 are first-class Amana plates

This board

Outdoor communicating control

  • Codes are on the outdoor board, not the furnace IFC. Do not diagnose this unit from an indoor E2/E9.
  • High pressure, low pressure, and comms (ComfortBridge pair only) are the usual three.
  • If the indoor is GR9T96 or GMEC96 (24V), this outdoor must be landed on Y1/Y2 — do not expect ComfortBridge menus indoor.
  • Communicating pair: data pair polarity, one master. Comm faults are invalid on a 24V-only hookup.

Sequence

  1. Y1 / low. Outdoor starts low capacity. Indoor blower on low/mid Cool tap (24V) or communicating CFM (ComfortBridge indoor).
  2. Y2 / high. Second stage. Head and subcooling should rise into the OEM SC window.
  3. Communicating pair. GMVC96 / AVPTC indoor requests capacity on the data pair. No conventional Y needed. A 24V GR9T96 on that same pair is a miswire.

Faults on this IFC

  • HPS

    High pressure

    lockout

    Outdoor airflow, overcharge, or restriction opened the high-pressure switch.

    • Outdoor fan running? Coil dirty, recirc, bent fins.
    • Subcooling vs indoor TXV sheet. Liquid-line drier / restriction.
    • Do not keep resetting HPS into a locked fan.
  • LPS

    Low pressure / freeze

    lockout

    Suction dropped out — charge, indoor airflow, or TXV.

    • Indoor filter, blower, coil freeze. Do not add gas on a frozen coil.
    • TXV / restriction. Weigh-out if the history is a leak.
    • Low ambient and wind can nuisance-trip LPS on a light load — still prove charge.
  • Comm

    Indoor/outdoor data

    lockout

    ComfortBridge data pair only. Invalid as a diagnosis on a 24V Y1/Y2 hookup.

    • Data pair polarity and splices. This is not 24V Y.
    • Indoor IFC family must be GMVC / AVPTC communicating. GR9T96 / GMEC96 / GMSS96 cannot be the ComfortBridge indoor.
    • One master on the bus. Outdoor board power before you condemn the indoor IFC.

Workflows

No cool, furnace is GR9T96

Mixed 24V GR9T96 (or GMEC96) indoor + GSXC outdoor.

  1. This pair is 24V. Y1 and Y2 at the outdoor. No ComfortBridge indoor. Do not look for indoor menus.
  2. Contactor / outdoor board 24 VAC, then pressures and outdoor fan.
  3. Charge by SC on a TXV coil. Superheat is the TXV health check, not the charge method.
  4. If someone ran ComfortBridge data wire to a GR9T96, that is a miswire — not a bad outdoor board. Land Y1/Y2/C or change the indoor to GMVC / AVPTC.
  5. Indoor cool tap still matters — a 9-speed left on a low cool tap will freeze and look like LPS.

Test points

Charge
Nameplate + line-set adders. SC per indoor TXV sheet.
Y1 / Y2
24V pair: both present on high stage. Communicating pair: data, not 24 VAC.
Outdoor board
LED/display before the disconnect. Codes die when you pull power.

Gotchas

  • ASXC18 / AVXC20 are Amana badges — use the Amana ASXC18 chart, do not stay here.
  • GSZC18 is the heat-pump sibling — reversing valve and defrost live on that chart, not here.
  • A GSX14 piston unit in the same yard is a different board. Do not apply this comms story to a contactor-and-cap condenser.
  • GLXT7C is the R-32 two-stage successor. Different refrigerant.

Related plates

  • amana-asxc18 — Amana-badge two-stage / comm outdoor. Own chart.
  • goodman-glxt7c — R-32 two-stage successor. Wrong refrigerant.
  • goodman-gxv9s — Goodman inverter AC. Not this two-stage scroll.
  • goodman-gszc18 — Heat-pump sibling. Reversing valve + defrost. Not this AC-only outdoor.
  • goodman-gmvc96 — ComfortBridge indoor that can actually talk to this outdoor on the data pair.
  • goodman-gr9t96 — 24V indoor mix. Y1/Y2 only — no ComfortBridge menus indoor.