Goodman GSZC18 Heat Pump Error Codes — What They Mean
The Goodman GSZC18 is an 18 SEER, two-stage heat pump designed for use with Goodman’s ComfortNet communicating system or a conventional thermostat. The outdoor control board flashes fault codes through the diagnostic LED. When paired with a ComfortNet thermostat, fault descriptions appear directly on the thermostat screen with more detail.
Goodman GSZC18 Flash Code Reference
| Flash Code | Meaning |
|---|---|
| 1 flash | High-pressure switch lockout |
| 2 flashes | Low-pressure switch lockout |
| 3 flashes | Outdoor ambient temperature lockout |
| 4 flashes | Compressor protection (internal overload) |
| 5 flashes | Defrost control fault |
| 6 flashes | Outdoor fan motor fault |
| 7 flashes | Low-stage high-pressure switch fault |
| 8 flashes | Control board fault |
| 9 flashes | Discharge temperature sensor fault |
Common Causes by Code
- Code 1 — High pressure lockout — On the GSZC18, the two-stage compressor runs at low capacity first. High-pressure trips at low stage are often caused by a dirty condenser coil or an airflow problem more severe than a standard single-stage unit. Confirm the coil is clean on all four sides (the GSZC18 uses a full-wraparound coil design).
- Code 2 — Low pressure lockout — Low refrigerant charge is the primary cause. The GSZC18 uses R-410A and has a specific charge requirement per the installation data plate. Also check the low-pressure switch contacts.
- Code 3 — Ambient lockout — The GSZC18 has a low-ambient cooling lockout (below 55°F by default) and a low-ambient heating lockout (at the compressor’s rated minimum). Code 3 in winter is normal operation; in summer, check the ambient sensor.
- Code 4 — Compressor protection — Two-stage compressors are more expensive to replace. If Code 4 appears frequently, check supply voltage (both legs), run capacitor values, and refrigerant charge before condemning the compressor.
- Code 5 — Defrost fault — The GSZC18 uses a time-temperature defrost board. A failed defrost termination sensor (clip-on type on the outdoor coil) is the most common cause. Also check the defrost relay on the control board.
- Code 6 — Fan motor fault — The GSZC18 may have an ECM or PSC outdoor fan motor depending on the model year. Check the motor capacitor, power supply, and motor winding continuity.
Step-by-Step Fix {#fix}
- Read the flash code — Access the control compartment on the side of the GSZC18 cabinet. The LED is on the control board.
- For Code 1 — Clean the condenser coil. Confirm fan rotation (air exits top). Check both high-pressure switch contacts with power off.
- For Code 2 — Connect gauges. R-410A suction pressure at 75°F ambient, cooling mode, should be approximately 110–115 PSI. Below 90 PSI indicates low charge. Perform electronic leak detection before adding refrigerant.
- For Code 4 — Let the unit cool for 30 minutes. Measure supply voltage at the disconnect under load — both legs to ground and leg-to-leg. Check the dual run capacitor with a capacitor meter (compare to nameplate µF rating ±6%).
- For Code 5 — Locate the defrost sensor on the outdoor coil (typically clipped to a U-bend near the bottom of the coil). Confirm it is making firm contact with the coil. Measure resistance and compare to the temperature-resistance chart in the service data.
- For Code 6 — With power off, spin the fan blade — should rotate freely. Measure capacitor µF. Check motor windings for continuity (common to each winding).
- Reset and monitor — After repair, cycle power and run the unit in both heating and cooling to confirm no fault recurrence.
Parts Often Needed
| Part | Notes |
|---|---|
| Dual run capacitor | Amazon | High failure rate in hot climates; check µF |
| Defrost sensor | Amazon | Clip-on; confirm coil contact |
| Defrost board | Amazon | If sensor is good but defrost won’t initiate |
| Contactor | Amazon | Pitted contacts cause compressor protection codes |
| Refrigerant (R-410A) | Amazon | Fix leak first; EPA certification required |
| Outdoor fan motor | Amazon | PSC or ECM depending on model year |
When to Call a Pro
Two-stage compressor diagnosis and refrigerant work require professional tools and certification. The GSZC18’s two-stage valve and ComfortNet communication system add complexity that makes board-level diagnosis difficult without Goodman’s diagnostic software.