Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Goodman GMS80 Furnace Error Codes — What They Mean
The Goodman GMS80 is an 80% AFUE single-stage gas furnace with a PSC blower motor. It is a straightforward, high-reliability unit widely used in residential applications and is one of the most common furnaces in service across North America. The GMS80 uses a single-LED diagnostic system on the Goodman control board — the same flash code system used across the Goodman/Amana/Daikin residential furnace lineup. The LED is visible on the board through the lower access panel window.
Goodman GMS80 LED Flash Code Reference
| Flash Count | Fault |
|---|---|
| 1 flash | Normal — system waiting for call |
| 2 flashes | Pressure switch stuck open |
| 3 flashes | Pressure switch stuck closed |
| 4 flashes | Open limit device (high-limit or aux limit) |
| 5 flashes | Flame sensed without gas valve call |
| 6 flashes | 6 ignition retries failed — hard lockout |
| 7 flashes | Gas valve (low-fire) fault |
| 8 flashes | Low igniter current detected |
| 9 flashes | Rollout switch open |
| Rapid flash | Low control voltage (below 19VAC) |
Common Causes by Code
- 2 flashes — Pressure switch open — On an 80% furnace like the GMS80, the most common cause is a blocked flue pipe or a bad inducer motor that isn’t creating enough draft negative pressure. Also check the pressure switch hose for cracks or separation at the inducer housing.
- 3 flashes — Pressure switch stuck closed — The switch is closing before the inducer starts. A condensate-filled pressure switch hose can keep the switch closed. Disconnect the hose and drain/blow it clear.
- 4 flashes — Limit open — Dirty filter, blocked supply or return, or a failed blower motor. The GMS80 high-limit switch opens around 160–180°F on the heat exchanger outlet. Check the temperature rise (should be 35°F–65°F for most GMS80 configurations).
- 6 flashes — Ignition lockout — After 6 failed ignition attempts, the GMS80 locks out and requires manual reset (press the board reset button or cycle power). Common causes: failed igniter, no gas, dirty flame sensor.
- 9 flashes — Rollout switch — Manual reset safety switch at the burner. Trips if flames escape the burner box. Do not operate the furnace until the root cause is found — typically a cracked heat exchanger or blocked flue.
Step-by-Step Fix
- Find the LED — Look through the lower access panel observation window on the GMS80. The LED is on the right side of the control board. Count the flashes (pause → flashes → pause = one code cycle).
- For 2 flashes (pressure switch open) — Disconnect the pressure switch hose at the inducer housing and at the switch. Blow air through the hose — should be clear. Also check the flue pipe run for bird nests or ice blockage (common in late fall/early spring).
- For 4 flashes (limit open) — Replace the air filter immediately. Check the return air temperature — should be between 65°F and 80°F entering the furnace. Also spin the blower wheel by hand (with power off) to confirm it’s not obstructed by debris (socks, plastic bags often get sucked into the return).
- For 6 flashes (ignition lockout) — Reset the board. Watch the ignition sequence through the sight glass: inducer on → igniter glow (visible orange in 17–25 seconds) → gas valve click → flame. If the igniter doesn’t glow brightly, measure its resistance (40–90 Ω cold is normal for silicon nitride).
- For 9 flashes (rollout) — Find and press the manual reset button on the rollout switch (red button at the burner compartment). Before resetting, look into the burner compartment — excessive carbon deposits indicate a heat exchanger blockage. An HVAC tech with a combustion analyzer should inspect before returning to service.
Parts Often Needed
| Part | Notes |
|---|---|
| Hot surface igniter | Goodman uses both silicon nitride and silicon carbide depending on production year |
| Flame sensor | Rod-type; clean before replacing |
| Pressure switch | Check hose first; switches fail after condensate contamination |
| High-limit switch | L160 or L180 depending on GMS80 variant |
| Rollout switch | L195°F or L270°F; manual reset |
| PSC blower capacitor | Single-run capacitor on PSC motor; check µF |
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 furnace 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
A Goodman GMS80 that trips the rollout switch (Code 9) requires professional inspection before return to service. A cracked heat exchanger allows combustion gases to mix with circulated air — a carbon monoxide risk. Do not bypass the rollout switch. If the furnace repeatedly trips the limit (Code 4) after filter replacement, a cracked heat exchanger or ECM motor issue may be the root cause requiring professional diagnosis.