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The Goodman furnace line uses a diagnostic LED on the control board to signal fault conditions. When six flash codes appear, it usually indicates one of several common issues that prevent the unit from safely operating. Because exact codes can vary by model and production year, technicians should refer to the specific model’s manual or the control board label for precise meanings. This article outlines six typical flash-code categories seen on Goodman furnaces, what each code commonly signals, and practical steps technicians can take to diagnose and address the problem. If the issue persists, professional service is advised.

Code 1 Flash: Ignition Or Lockout

What the code typically signals: The furnace attempted to ignite but failed to establish a stable flame, or the flame was not detected after the ignition sequence, triggering a lockout.

• Common causes: Insufficient gas supply, gas valve closed, weak or cracked igniter, dirty flame sensor, thermostat signal timing issues, or faulty ignition control.
• What to check: Confirm gas supply is on and the gas valve is open; inspect the igniter for damage or cracking; clean or inspect the flame sensor; verify ignition sequence timing and wiring integrity between the thermostat, control board, and ignition module.
• When to escalate: If there is no gas smell, if the igniter shows visible damage, or if the unit repeatedly fails to ignite after several attempts, a professional should diagnose gas supply and ignition circuitry.

Addressing ignition lockout often begins with ensuring gas availability and a clean flame sensor. If the problem persists after addressing these basics, a service technician should test the ignition module and control board for faults.

Code 2 Flashes: Gas Supply Or Valve Issue

This code commonly points to problems in delivering gas to the burner or in the gas-valve operation itself.

• Common causes: Gas valve not opening, low gas pressure, regulator fault, or a shut-off due to an earlier fault; occasional thermostat miscommunication causing delayed gas flow.
• What to check: Verify the gas supply line is clear and the valve is fully open; confirm adequate gas pressure at the furnace; inspect the gas valve and its wiring; ensure the thermostat is signaling correctly to request heat.
• When to escalate: If gas pressure is inadequate or a valve fails to actuate, a licensed HVAC technician should service the gas train and related controls.

Gas-related codes require caution. Do not attempt gas-line work without proper qualifications. After correcting supply or valve issues, the control board should be reset according to the manufacturer’s instructions and the unit retested.

Code 3 Flashes: Flame Sensor Malfunction

Code 3 typically indicates a flame-sensing issue where the flame is not detected despite ignition.

• Common causes: Dirty or misaligned flame sensor, faulty flame sensor wiring, or a weak flame due to improper gas combustion.
• What to check: Inspect and gently clean the flame sensor with a non-abrasive cloth; inspect sensor alignment and mounting; examine sensor wiring for corrosion or loose connections; verify the burner and igniter are producing a stable flame.
• When to escalate: If cleaning and alignment do not restore flame sensing, a technician should test the sense circuit, the ignition module, and related control wiring.

Flame-sensing issues can lead to repeated lockouts. Proper sensor function is essential for safe operation, so professional inspection may be required if sensor replacement or wiring repair is needed.

Code 4 Flashes: Inducer Or Venting Problem

Code 4 often points to ventilation or draft issues that prevent the furnace from exhausting combustion gases properly.

• Common causes: Blocked venting, restricted air flow, dirty inducer wheel, or a misread pressure switch due to poor venting or air leaks.
• What to check: Inspect the intake and exhaust vents for obstructions; listen for inducer operation during the start-up sequence; check the pressure switch hose and connections for leaks; verify seals around the furnace and vent system.
• When to escalate: If the inducer runs but the pressure switch does not close, or if venting appears blocked, a professional should inspect venting integrity and the inducer assembly.

Venting problems can cause carbon monoxide risks. Do not bypass safety checks; contact a qualified HVAC technician to verify combustion safety and vent integrity.

Code 5 Flashes: High Temperature Or Limit Switch

Code 5 signals that the furnace has overheated or the high-limit switch has tripped, cutting power to the furnace to prevent damage.

• Common causes: Restricted airflow due to a dirty filter, blocked return-air pathways, closed registers, or a failing blower motor; ambient temperatures and ductwork design can contribute to overheating.
• What to check: Replace or clean the air filter; inspect supply and return air paths for obstructions; ensure registers are open and ducts are clear; test the blower motor and belt condition if applicable.
• When to escalate: If the limit switch repeatedly trips after airflow issues are resolved, a technician should evaluate electrical load, blower operation, and potential faults in the heat exchanger or sensors.

Consistent overheating can indicate airflow problems or component wear. A professional should confirm safe operation and perform any required repairs or replacements.

Code 6 Flashes: Control Board Or Sensor Failure

Code 6 generally points to a fault within the control board circuitry or a malfunctioning sensor that communicates with the board.

• Common causes: Faulty thermostat communication, defective control board, sensor malfunctions, or loose wiring harness connections.
• What to check: Inspect thermostat wiring and signals; verify connector integrity on the control board; look for visible burnt components or corrosion; ensure all harnesses are securely connected.
• When to escalate: If basic wiring checks do not reveal an obvious issue, a professional should diagnose the control board and sensors, as replacement may be necessary.

Control-board failures can affect multiple furnace functions. If other codes are present or if troubleshooting does not restore operation, professional diagnosis is recommended to prevent misdiagnosis or further damage.