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Models
Goodmanac

GSXC18

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).

Diagnose this modelInstall notesBoard cross-refField wiring

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.

Outdoor LED / display before you pull the disconnect — disconnect clears it.

Universal swap

No universal

Outdoor communicating condensing unit. No furnace IFC universal.

White-Rodgers

—

ICM

—

Honeywell

—
  • Indoor furnace universals (50A55, 50M56U, ICM282B, ICM2811). This is not an IFC.
  • Do not ‘adapt’ a furnace board to an outdoor inverter or defrost PCB.

After you pull the dead card

  1. 1OEM outdoor control. This is not a 50A55 / ICM282B job.

Field wiring

Written landings for Goodman GSXC18. Not a factory schematic.

ComfortBridge two-stage outdoor
1/2 data — or 24V Y1/Y2R-410A

Goodman GSXC / Amana ASXC class. Talks ComfortBridge on 1 and 2 when the indoor is GMVC / AMVC / AVPTC. On a 9-speed 24V indoor it is just Y1/Y2.

Written circuit description only — FieldBench does not redraw maker schematics. Example: R to W closes on a heat call; verify 24V at W1 before you change the IFC.

Open manufacturer PDF

This indoor

GMVC / AVPTC on 1/2. GR9T96 / GMEC96 = Y1/Y2 only.

ComfortBridge indoor

GMVC96 / AMVC96 / AVPTC / ARVT96 on the data pair.

CoolCloud / communicating control

Menus live on the indoor. Do not land R/C on 1 and 2.

  • 1DataData
  • 2DataData

Communicating indoor

One master on the bus. 1/2 polarity matters.

  • 1DataData
  • 2DataData
  • R24V for accessories24V
  • CCommon24V

ComfortBridge pair

Indoor 1 and 2 to outdoor 1 and 2. Polarity matters. This is not 24V Y.

  • 1DataData
  • 2DataData

Outdoor line

208/230 through the disconnect to the contactor line side.

  • L1LineLine
  • L2LineLine
  • GNDEquipment groundLine

Landing

  • DataIndoor 1outdoor 1 · indoor 2outdoor 2

    Data. Not 24 VAC.

  • LineL1 / L2outdoor

    208/230

24V indoor (GR9 / AR9 / GMEC)

9-speed or CT indoor. No ComfortBridge menus.

Wall thermostat

Two-stage cool. A single-Y stat never pulls high.

  • R24V hot24V
  • C24V common24V
  • Y1Low-stage cool24V
  • Y2High-stage cool24V
  • GIndoor fan24V
  • WHeat. Stays at the indoor.24V

Indoor IFC / AHU

Y1 and Y2 both have to leave this cabinet on high.

  • RTransformer hot24V
  • CCommon — run it to the outdoor24V
  • Y1Low-stage in / out24V
  • Y2High-stage in / out24V
  • GBlower24V
  • W1Heat24V

Outdoor — Y1 / Y2

Low vs high is these two screws, not an inverter L-code.

  • Y1Low-stage request24V
  • Y2High-stage request24V
  • C24V common24V

Outdoor line

208/230 through the disconnect to the contactor line side.

  • L1LineLine
  • L2LineLine
  • GNDEquipment groundLine

Landing

  • 24VStat Y1/Y2/Cindooroutdoor Y1/Y2/C

    ~24 VAC. Data wire is a miswire.

Prove it

  • Identify the indoor first. Communicating indoor = 1/2. 24V indoor = Y1/Y2.
  • Comm faults are invalid on a 24V hookup.

Do not

  • Do not land data wire on a GR9T96 / AR9T96 / GR9S96.
  • Do not apply this to GSX16 / GLXS4B or GXV9S / AXV9S.

1 and 2 are the published ComfortBridge pair.

Sequence

  1. 1

    Y1 / low

    Outdoor starts low capacity. Indoor blower on low/mid Cool tap (24V) or communicating CFM (ComfortBridge indoor).

  2. 2

    Y2 / high

    Second stage. Head and subcooling should rise into the OEM SC window.

  3. 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

Step 1, then 2, then 3. Expected reading and the fail branch sit under each check.

HPS

High pressure

lockout

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

  1. 1 Heat rejection first

    On Goodman GSXC18 high pressure is outdoor airflow until proven otherwise. Fan running the right direction, coil clean, fins not folded, no recirc over a fence or through a garage. Outdoor fan running? Coil dirty, recirc, bent fins.

    Expect: OD fan on, coil clean, discharge air leaving the top/side freely.

  2. 2 Head, subcooling, and liquid line

    Liquid pressure and liquid-line temperature → SC. High head + high SC = overcharge or non-condensables. High head + low SC = restriction or a dying compressor. Do not add gas because suction is low. Subcooling vs indoor TXV sheet. Liquid-line drier / restriction.

    Expect: SC vs the metering device on this plate (TXV ≠ piston).

  3. 3 Restriction vs the switch

    Drier, liquid-line kink, TXV inlet screen, or a non-feed indoor coil will stack head. Ohm / jumper-test the HPS only as a momentary prove, then restore it. A switch that opens on a known-good charge and clean coil is the switch. Do not keep resetting HPS into a locked fan.

    Expect: HPS closed at a normal liquid pressure for this refrigerant and OD ambient.

LPS

Low pressure / freeze

lockout

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

  1. 1 Airflow and freeze before you add gas

    On Goodman GSXC18 low pressure / freeze is indoor airflow or a starved coil until proven otherwise. Filter, blower, dirty A-coil, closed supplies. In heat mode, a iced outdoor coil is the same family. Indoor filter, blower, coil freeze. Do not add gas on a frozen coil.

    Expect: Indoor air moving. Coil not iced. OD coil clear in heat.

  2. 2 SH / SC after a thaw

    Thaw fully. Then SH and SC vs the metering device. Low charge ices a piston. A TXV stuck closed ices with high SH. A restriction downstream of the ports fakes a low-charge chart. TXV / restriction. Weigh-out if the history is a leak.

    Expect: SH/SC in range for this refrigerant and device after a stable 10-minute run.

  3. 3 Leak, TXV, then the LPS

    Weigh-in if the chart says undercharge. Soap / electronic leak on joints and the coil. LPS that opens on a known-good charge and a clear coil is the switch — momentary test only, then restore. Low ambient and wind can nuisance-trip LPS on a light load — still prove charge.

    Expect: Charge on the nameplate. LPS closed at a normal suction for this OD ambient.

Comm

Indoor/outdoor data

lockout

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

  1. 1 Data pair, not 24 V

    On Goodman GSXC18 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 and splices. This is not 24V Y.

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

  2. 2 Power both ends, then the outdoor status

    Indoor transformer and outdoor control both alive. Read the outdoor LED / 7-seg. Water in the outdoor control is the usual dead-end after a storm. Indoor IFC family must be GMVC / AVPTC communicating. GR9T96 / GMEC96 / GMSS96 cannot be the ComfortBridge indoor.

    Expect: Both controls powered. Outdoor showing a comm or address state, not a blank board.

  3. 3 Address / bias, then the control

    Communicating plates need a matching indoor address and sometimes a terminating bias. Swap is last: known-good pair + power both ends + correct address. One master on the bus. Outdoor board power before you condemn the indoor IFC.

    Expect: Indoor sees the outdoor. No comm code after a power cycle.

Workflows

No cool, furnace is GR9T96

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

  1. 1 Prove the call and the live code

    This pair is 24V. Y1 and Y2 at the outdoor. No ComfortBridge indoor. Do not look for indoor menus.

    Expect: Write whatever Outdoor communicating control is showing. Outdoor LED / display before you pull the disconnect — disconnect clears it.

    If fail: No call at the IFC → stat / harness. Do not change gas parts.

  2. 2 Isolate the first failed step in the sequence

    Contactor / outdoor board 24 VAC, then pressures and outdoor fan.

    Expect: Y2 / high: Second stage. Head and subcooling should rise into the OEM SC window.

  3. 3 Confirm with a number, then the next part

    Charge by SC on a TXV coil. Superheat is the TXV health check, not the charge method. 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. Indoor cool tap still matters — a 9-speed left on a low cool tap will freeze and look like LPS.

    Expect: 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.

    If fail: Stay on this plate. The out-of-range reading is the part.

    If pass: Reading in range → the next step in this workflow is the repair.

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

Install this plate

Install GSXC18 as a two-stage / communicating AC outdoor. Not a piston GSX16.

Outdoor AC · two-stage or two-step · TXV indoor required for rating · ComfortBridge pair or 24V Y1/Y2

Lines & charge

  • TXV indoor. Do not leave a piston in the coil and expect the rating or a clean SH.
  • Weigh-in per the outdoor nameplate and line-set adder. Two-stage scroll is not a ‘charge to 10 °F SC’ piston unit.
  • Nitrogen braze, evacuate to 500 microns, weigh in. No compressor start until the indoor TXV is confirmed.

Wiring

  • ComfortBridge data pair to GMVC / AVPTC / AMVC, or 24V Y1/Y2 from a conventional furnace.
  • A GR9T96 indoor is 24V only — use Y1/Y2, not a data pair that furnace does not have.

First fire

  1. 1Y1 only, then Y2. Confirm the outdoor actually steps.
  2. 2SH/SC to the GSXC18 chart with a TXV indoor — not a GSX16 piston target.

Do not on Goodman GSXC18

  • Do not apply GSZ14 defrost / RV steps. This outdoor has no reversing valve.
  • Do not look for furnace E-codes on this cabinet.

Maker literature

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Warranty and bulletins

Affected models, serial range, symptom, remedy, date. Maker files open on the manufacturer site.

All bulletins

  • Dealer portals

    dealer

    Where remaining service letters live

    Current-year service letters not on a public literature page · Maker dealer portal

    Open factsOpen manufacturer PDF