Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Goodman GMSS96 Furnace Error Codes — What They Mean
The Goodman GMSS96 is a 96% AFUE, single-stage, multi-speed gas furnace. It is a high-efficiency upflow/horizontal unit with a PVC flue and condensate drain system. Faults are reported through a diagnostic LED on the integrated furnace control board — flash sequences identify specific fault conditions. The LED is visible through the sight glass on the lower access door.
Goodman GMSS96 Flash Code Reference
| Flash Code | Meaning |
|---|---|
| Continuous on | No call for heat (normal) |
| Continuous off | No power to board |
| 1 flash | Lockout — gas or ignition fault |
| 2 flashes | Pressure switch stuck open |
| 3 flashes | Pressure switch stuck closed |
| 4 flashes | Open high-limit device |
| 5 flashes | Flame sensed, gas valve not energized |
| 6 flashes | Rollout switch open |
| 7 flashes | Inducer motor fault |
| 8 flashes | Low flame signal |
| 9 flashes | Reversed 115V AC polarity |
| 10 flashes | Gas valve circuit fault |
Common Causes by Code
- Code 1 — Lockout — The GMSS96 locks out after three failed ignition attempts or if flame is lost three times within one heating cycle. Check the ignitor, flame sensor, and gas pressure before resetting.
- Code 2 — Pressure switch stuck open — Common on the GMSS96 because of its dual heat exchanger design. The secondary heat exchanger collects condensate; a plugged drain causes the pressure switch to see positive (instead of negative) pressure and fail to close. Always check the PVC condensate drain first.
- Code 3 — Pressure switch stuck closed — The pressure switch contacts should open when the inducer is off. If they remain closed, the board won’t start the inducer. A failed switch or a stuck contact is the usual cause.
- Code 4 — High limit — The GMSS96 has both a main limit switch and auxiliary limit. Dirty filter, blocked ductwork, or a failed blower motor are the primary causes. The auxiliary limit is on the secondary heat exchanger and trips if secondary drain blockage causes condensate to back up against it.
- Code 6 — Rollout — Manual-reset rollout switches on the GMSS96 are on the burner manifold. Rollout indicates heat exchanger crack, incorrect gas pressure, or burner blockage.
- Code 7 — Inducer fault — The GMSS96 monitors inducer operation. If the inducer fails to start or stalls, Code 7 appears. Confirm 120V at the inducer motor, then test winding continuity.
- Code 8 — Weak flame — Low µA signal from flame sensor. Clean the sensor rod with emery cloth. If signal remains below 1.0 µA, check ground continuity of the sensor wire back to the board.
Step-by-Step Fix
- Read the LED — The GMSS96 LED is on the lower control board. Count flashes before opening anything.
- For Code 1 — Reset by cycling the power off for 30 seconds. Then monitor the next startup cycle closely: does the ignitor glow? Does gas ignite? Does the flame establish?
- For Code 2 (pressure switch open) — Disconnect the drain line at the trap and blow through it to confirm it’s clear. Check all PVC condensate fittings for cracks. Shake the pressure switch — if you hear liquid inside, condensate has entered the switch body (replace it).
- For Code 4 — Check both the main and auxiliary limit switch continuity with a multimeter. The auxiliary limit is located inside the unit near the secondary heat exchanger. A tripped auxiliary limit without a blower problem suggests drain backup against the limit.
- For Code 7 — Check inducer motor: confirm it attempts to start, listen for humming without rotation (failed capacitor), or complete silence (open motor winding or no power).
- For Code 8 — Remove flame sensor, clean rod with emery cloth, reinstall. Measure DC microamps in series with the sensor lead to the board. Normal is 1.5–4.0 µA.
- Reset and test — After any repair, cycle the power switch, set the thermostat to heat, and run one full cycle.
Parts Often Needed
| Part | Notes |
|---|---|
| PVC condensate trap | Specific to GMSS96; replace if cracked or plugged |
| Flame sensor | Common; OEM or universal replacement |
| Pressure switch | Match water column rating to model (0.85” or 1.5”) |
| Inducer motor | With or without housing; check wheel for debris first |
| High-limit switch | Check both main and auxiliary |
| Rollout switch | Investigate crack before replacing |
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
The GMSS96’s secondary heat exchanger system is complex. Condensate-related failures can cascade into board damage if left undiagnosed. If you see repeated Code 4 trips with a clean filter, or if the unit produces a burning smell, call a licensed technician — do not operate the furnace until the secondary heat exchanger is confirmed intact.