FieldBench

Goodman

GSZC18

Heat pump · Two-stage / communicating heat pump outdoor

Goodman/Amana two-stage or communicating heat pump outdoor (GSZC18 / GSZC16 / ASZC18). Reversing valve plus defrost control live on the outdoor board. Codes are outdoor, not furnace E-codes. GSZB16 is listed as a search alias only — many GSZB units are single-stage with a simpler defrost board.

Not GSXC18 (AC-only, no reversing valve, no defrost). Not GSX14 piston AC. GSZB14 / GSZB16 single-stage heat pumps use a simpler time/temp defrost board — do not apply this communicating / two-stage outdoor chart to those DST boards.

Walk this in Diagnose

This plate

Stages
Two-stage (GSZC) · GSZB single-stage cousins use a different defrost board
Reversing valve
Present. Energize direction is board-specific — read the outdoor diagram
Defrost
Timed / demand on the outdoor control (not a furnace IFC function)
Pair
ComfortBridge indoor (GMVC / AVPTC) or 24V Y1/Y2 + O/B
Sister SKU
GSZC16 · Amana ASZC18 same family

This board

Outdoor heat-pump control · defrost + RV + pressure

  • Codes live on the outdoor board. Indoor furnace E2/E9 is a different chart.
  • HPS / LPS are pressure. Defrost-sensor and ‘stuck in defrost’ are this board, not the indoor IFC.
  • Comm / U-style codes are valid only on a ComfortBridge pair. On 24V Y1/Y2 they mean a miswire.
  • Write the outdoor LED/display before you pull the disconnect.

Sequence

  1. Y1 cool. Outdoor runs cool capacity. Indoor blower on cool. Reversing valve in the cool position per THIS outdoor diagram.
  2. O/B reversing valve. Read the outdoor diagram. Do not assume Carrier O-on-cool. Goodman boards have shipped O energized in cool on some and B energized in heat on others. Confirm B vs O on the board silkscreen / IO before you swap a stat or an air handler.
  3. Heat (HP). Y (and O/B as required) at the outdoor. Compressor and outdoor fan in heat. Indoor is the condenser. If the outdoor is running in cool while the house wants heat, the RV did not shift or O/B is wrong.
  4. Defrost timed / demand. Outdoor board terminates defrost on coil sensor and/or time. Sensor open/short or a failed board leaves the unit iced or stuck in defrost (outdoor fan off, RV in cool, steam / water under the unit for too long).

Faults on this IFC

  • HPS

    High pressure

    lockout

    Discharge / liquid pressure opened the high-pressure switch. In heat this is often indoor airflow; in cool this is outdoor airflow or overcharge.

    • Mode first. Heat-mode HPS = indoor coil, filter, blower, TXV. Cool-mode HPS = outdoor fan/coil.
    • SC / SH in the running mode. Restriction vs overcharge.
    • Fan running in that mode? A unit stuck in defrost will look like a cool-mode HPS if you are not watching the RV.
  • LPS

    Low pressure

    lockout

    Suction dropped out — charge, airflow in the evaporating coil, or a coil iced over in heat.

    • Heat mode: outdoor coil iced, outdoor fan, defrost not running. Do not add gas on an iced outdoor.
    • Cool mode: indoor airflow, freeze, TXV.
    • Weigh-out if the history is a leak. LPS on a heat pump is not automatically ‘low on 410A.’
  • Defrost sensor / stuck in defrost

    Defrost path

    lockout

    Outdoor coil sensor open/short/out of range, or the board will not leave defrost (fan off, RV shifted to cool, long steam).

    • Ohms the outdoor coil / defrost sensor vs the board chart. Unplugged sensor is a stuck-defrost classic.
    • Board initiating defrost on time with a warm coil? Bad sensor or bad board — prove the sensor first.
    • Outdoor fan must restart when defrost terminates. A dead fan motor ices the coil again on the next heat call.
  • RV / no shift

    Reversing valve / no shift

    lockout

    Unit runs the opposite mode, or pressures never swap when O/B is commanded.

    • Voltage at the RV solenoid during the call that should energize it. Confirm O vs B on THIS outdoor diagram — do not assume Carrier O-on-cool.
    • Solenoid ohms / mechanical valve stuck. Tap test is not a repair.
    • Suction line temp and gauge swap are the prove. Indoor blowing cool on a heat call with the outdoor running is RV or O/B until proven otherwise.
  • U / Comm

    ComfortBridge / U-style comm

    lockout

    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.

    • 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.
  • Outdoor fan

    Failed outdoor fan

    lockout

    Fan not running in cool, or not restarting after defrost in heat. Ices the outdoor in heat; high-heads in cool.

    • Voltage at the fan motor / ECM plug when the board is commanding fan.
    • Capacitor only on PSC fans — many of these outdoors are ECM. Confirm the motor family.
    • Blade ice, seized bearing, control output. A fan that dies mid-defrost leaves a block of ice and an LPS/HPS sandwich.

Workflows

No heat pump heat

Indoor calling heat, outdoor running in cool or not shifting, house cold.

  1. Confirm the indoor is actually asking the outdoor for HP heat — not W-only furnace heat with Y open.
  2. Read O vs B on the outdoor diagram. A Carrier-wired O-on-cool stat on a B-on-heat Goodman board runs the RV backwards.
  3. Voltage at the RV solenoid on the call that should energize it. No voltage → board / O-B landing. Voltage, no shift → valve.
  4. Outdoor running, indoor blowing cool: RV or O/B. Outdoor silent: power, HPS/LPS, comm.
  5. Defrost board stuck in defrost looks like ‘always cool at the outdoor.’ Sensor and terminate before you condemn the RV.

Iced outdoor

Outdoor coil is a block of ice in heat. LPS / no heat / water under the unit.

  1. Do not add charge on an iced coil. Recover the picture: fan, sensor, board, time in defrost.
  2. Defrost sensor ohms and placement on the coil circuit, not hanging in air.
  3. Does the board enter defrost (RV shift, outdoor fan stop, steam)? No → sensor/board. Yes, but fan never returns → fan motor / output.
  4. Airflow across the outdoor (snow, leaves, fence recirc) will re-ice a healthy defrost.
  5. Low charge ices a heat pump outdoor too — weigh after it is defrosted, not while it is a block.

U4 / comm with a 24V furnace indoor

GSZC on data wire into a GR9T96 / GMEC96 / GMSS96.

  1. Those indoor IFCs are not ComfortBridge. U / comm will never clear on data wire into a 9-speed or CT 24V board.
  2. Land Y1/Y2, O or B per the outdoor diagram, C, and common. Or change the indoor to GMVC96 / AVPTC.
  3. Do not replace the outdoor board to ‘fix U4’ on this mix. The indoor cannot speak the bus.
  4. After you convert to 24V, confirm O vs B again. The communicating indoor used to own that decision.

Test points

O vs B
Per the outdoor diagram on THIS unit. Do not assume Carrier O-on-cool.
Defrost sensor
Ohms vs outdoor-board chart. Mounted on the coil circuit.
Charge
Nameplate + line-set. SC in cool on a TXV indoor. Weigh-in after a defrost, not into ice.
Data pair
Only on ComfortBridge indoors. Never 24 VAC on the data pins.

Gotchas

  • GSZB14/16 single-stage often uses a small pin-style defrost board. Same yard, different chart.
  • ASZC18 is the Amana badge twin of GSZC18.
  • Amana AVPTC is the communicating air handler that actually belongs on the ComfortBridge pair with this outdoor.
  • Furnace W heat with the HP also calling is a dual-fuel / control issue — not an iced outdoor by itself.

Related plates

  • goodman-gsxc18 — AC-only sibling. No RV, no defrost. Same two-stage / comm outdoor idea.
  • amana-avptc — Communicating air handler that can be the ComfortBridge indoor for this HP.
  • goodman-gmvc96 — Communicating furnace indoor. 24V GR9T96 on this outdoor is Y1/Y2 + O/B, not data.