Temperature sensor
Common error-code part. Confirm resistance and connector style.
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
- 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 | High failure rate in hot climates; check µF |
| Defrost sensor | Clip-on; confirm coil contact |
| Defrost board | If sensor is good but defrost won’t initiate |
| Contactor | Pitted contacts cause compressor protection codes |
| Refrigerant (R-410A) | Fix leak first; EPA certification required |
| Outdoor fan motor | PSC or ECM depending on model year |
Technician Tips
Goodman furnaces use a simple LED flash system on the control board. The diagnostic LED is usually visible through the blower compartment window. Count the flashes between pauses — that’s your error code. One flash = normal operation. Two flashes = pressure switch fault. Three flashes = limit switch. Four flashes = flame sensor. Goodman flame sensors are notorious for carbon buildup — cleaning with emery cloth every 2-3 years prevents most lockout calls. The control board on GMS8 and GMSS9 models is particularly sensitive to voltage fluctuations — if you see multiple different codes, check the transformer output (should be 24VAC ±10%).
Prevention: Install a whole-house surge protector on the furnace circuit to prevent board damage from power spikes.
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
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.