When the temperature drops below freezing and a Goodman residential furnace locks out on a service call, speed and precision are critical. Modern Goodman furnaces use integrated control boards with diagnostic LED flash codes that tell you exactly where in the sequence of operation the system failed.

Here is our practical, field-proven troubleshooting guide for diagnosing draft inducer blowers, pressure switches, and gas valves on Goodman 80% and 90%+ AFUE furnaces.

1. Sequence of Operation Breakdown

Before testing individual parts, verify where the furnace stalls in its ignition sequence:

1. Call for Heat (W1 closed): Control board performs internal safety checks.
2. Draft Inducer Energizes: Inducer motor ramps up to create negative draft pressure through the heat exchanger.
3. Pressure Switch Closes: Diaphragm senses differential pressure (e.g. -0.65" W.C.) and signals the board.
4. Hot Surface Igniter Glows: Silicon nitride igniter energizes for 15–20 seconds.
5. Gas Valve Opens: 24VAC is sent to the gas valve solenoid; gas flows across burners.
6. Flame Sensor Proves Fire: Microamp rectification signal (0.5 – 5.0 µA) confirms steady ignition.

2. Troubleshooting 3 Flashes: Pressure Switch Stuck Open

A 3-flash error code is one of the most common Goodman furnace service calls. It does NOT automatically mean the pressure switch is defective. It means the control board did not see the switch contacts close after the draft inducer started.

• Step 1: Check the Flue & Condensate Drain: On 90%+ condensing units, a clogged condensate trap or blocked PVC exhaust vent prevents proper draft.
• Step 2: Inspect the Inducer Port: Use a small drill bit or wire to clear calcium and corrosion from the small brass barb where the rubber hose connects to the inducer housing.
• Step 3: Test Draft Inducer Motor (Goodman 66005): Measure voltage at the inducer molex plug (should be 120VAC). Check amp draw against the motor nameplate rating. If the motor hums, overheats, or shows open winding resistance, replace the inducer assembly.

3. Diagnosing Gas Valve Failures (Goodman 36G22-254)

If the igniter glows cherry red for 20 seconds and then shuts down without ignition, the issue is within the gas supply, valve solenoid, or control board relay.

• Check 24VAC Signal: Place multimeter leads on the two gas valve terminals during the ignition window. If 24VAC is present but the valve does not click or open, the valve solenoid is bad.
• Check Inlet and Manifold Pressure: Using a digital manometer, verify inlet natural gas pressure is between 5.0" and 7.0" W.C. When firing, adjust the manifold regulator screw to 3.5" W.C. for standard natural gas (or 10.0" W.C. for LP conversion).

Safety Warning: Always perform a combustible gas leak check using soapy bubble solution or an electronic leak detector on all pipe unions and valve fittings after replacing a gas valve.

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