Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Goodman 5 Flash Error Code — What It Means
Five flashes on a Goodman furnace diagnostic LED indicates “Flame Sensed When No Flame Should Be Present” — the flame sensor is detecting a signal during a period when the gas valve is closed and no burner should be lit. This is a safety lockout. The control board interprets a voltage signal from the flame sensor without a corresponding call for heat as a dangerous condition (potential gas leak igniting outside the normal sequence) and shuts the furnace down.
Common Causes
- Leaking or sticking gas valve — A gas valve that does not fully close allows a small amount of gas to reach the burners, which can ignite from residual heat and produce a flame signal.
- Contaminated or shorted flame sensor — A sensor rod coated with oxidation or silicate deposits may produce a false microamp reading even without flame present.
- Wiring short to the flame sensor circuit — A pinched or chafed wire between the sensor and the control board can induce voltage on the sensor circuit, mimicking a flame signal.
- Faulty control board — Rarely, the board’s flame-sensing circuit malfunctions and reports flame when none exists.
Step-by-Step Fix
- Turn off gas supply — Shut the manual gas shutoff valve upstream of the furnace before any inspection. Confirm no gas odor is present before proceeding.
- Inspect and clean the flame sensor — Remove the flame sensor rod (one screw, quarter-inch connector). Clean the rod with fine steel wool or 400-grit emery cloth. Reinsert and tighten.
- Check flame sensor wiring — Trace the wire from the flame sensor to the control board. Look for any areas where insulation is worn, pinched by a panel edge, or melted. Repair or replace the wire as needed.
- Test the gas valve — With gas on and the furnace in standby (not calling for heat), use a combustible gas detector near the burner compartment. Any gas reading indicates a leaking valve that must be replaced.
- Reset and test — Restore power, restore gas, and run a full heat cycle. If the 5-flash code returns immediately after burner shutoff (no call for heat), and the flame sensor and wiring are clean, replace the gas valve or control board.
Parts Often Needed
| Part | Notes |
|---|---|
| Flame sensor rod | Inexpensive universal or OEM; clean before replacing |
| Gas valve | Replace if leaking through in standby; match BTU rating |
| Flame sensor wire lead | Replace if insulation is damaged |
| Control board | Last resort after other components verified |
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
Any suspected gas valve leak requires a licensed gas technician. Do not operate the furnace if you smell gas or a combustible gas detector reads positive near the burner — evacuate and call your gas utility.
See Also
- Goodman Furnace E1 Error Code — Causes & Fix
- Goodman 7 Flash Error Code — Ignition Lockout Fix
- Goodman 3 Flash Error Code — Pressure Switch Stuck Open Fix
- Goodman 9 Flash Error Code — Causes & Fix