Modern furnaces have a built-in diagnostic system that communicates error codes through a flashing LED light on the control board. Instead of calling a technician immediately, many homeowners can diagnose the issue themselves by reading these blink patterns. Here's how.
Where Is the Diagnostic LED?
The diagnostic LED is located on the furnace control board, visible through a small sight glass (clear plastic window) in the lower access panel of your furnace, or directly visible when you remove the lower panel. It's typically a small amber, green, or red LED.
If your furnace isn't heating, look for this LED and count the blink patterns — the number and rhythm of blinks correspond to specific fault codes documented in your furnace's manual.
How to Read Blink Codes
Most furnaces use a sequence like: pause — [X blinks] — pause — [Y blinks] — repeat. The first number is typically the "error category" and the second number specifies the fault. For example, "3 blinks + 4 blinks" = fault code 34 on some Goodman models.
Other furnaces simply blink a single number repeatedly. Always count carefully and compare against the fault code chart, which is usually printed on a sticker inside your furnace's service panel door.
Common Goodman/Daikin Fault Codes
Goodman furnaces (including those built after the Daikin acquisition in 2012) typically use the following LED blink patterns on PCBBF112, PCBBF123, and related control boards:
| LED Pattern | Fault Code | Meaning | Common Cause |
|---|---|---|---|
| Steady ON | Normal | System operating or in standby | No fault |
| Steady OFF | No power | No 24V control power | Tripped breaker, blown fuse, wiring issue |
| 2 blinks | 02 | System lockout (ignition failed) | Failed ignitor, gas supply issue, flame sensor dirty |
| 3 blinks | 03 | Draft pressure error | Blocked flue, failed inducer motor, pressure switch stuck |
| 4 blinks | 04 | Open high limit | Dirty filter, blocked return air, failed limit switch |
| 5 blinks | 05 | Flame sensed without call for heat | Gas valve stuck open (serious — call immediately) |
| 6 blinks | 06 | 115V power polarity reversed | Incorrect wiring — electrical issue |
| 7 blinks | 07 | Low flame sense signal | Dirty/coated flame sensor rod, weak flame |
| 8 blinks | 08 | Ignitor circuit problem | Failed hot surface ignitor, wiring issue |
| 9 blinks | 09 | Rollout switch open | Blocked flue causing combustion rollout — serious hazard |
| 10 blinks | 10 | Low inducer speed | Aging inducer motor, blocked intake |
Note: Codes vary by model and control board version. Always verify against the sticker inside your furnace door panel or your specific model's installation manual.
The Most Common Codes — What to Do
2 Blinks — Ignition Lockout
This is the most common fault. The furnace tried to ignite 3 times and failed. Common causes:
- Failed hot surface ignitor — see our ignitor replacement guide
- Dirty flame sensor — clean with fine steel wool or emery cloth (non-abrasive)
- Gas supply problem — check that the gas shutoff is open and your other gas appliances work
Try resetting the furnace: turn the thermostat to OFF, wait 30 seconds, turn back to HEAT. If it faults again within a few cycles, a tech visit is warranted.
3 Blinks — Draft/Pressure Fault
The pressure switch didn't close, meaning the inducer motor isn't creating adequate draft. Check:
- Furnace flue pipe for blockages (bird nests, ice, debris at the exterior termination)
- PVC condensate drain — if blocked, it can trigger a pressure switch fault
- The plastic pressure switch hoses for cracks or disconnections
4 Blinks — High Limit Tripped
The heat exchanger got too hot. Almost always caused by restricted airflow:
- Check your furnace filter — if it's severely clogged, this is the cause
- Check that all supply and return registers are open
- Check that the blower motor is spinning when the furnace runs
9 Blinks — Rollout Switch
This is a safety fault that requires professional attention. A rollout condition means combustion gases are escaping the burner compartment — potentially into your living space. Do not reset and ignore this code. Call an HVAC technician.
When to DIY vs. Call a Tech
| Fault Code | DIY Friendly? | Notes |
|---|---|---|
| 2 blinks (ignition) | Yes — if ignitor/flame sensor | Ignitor replacement is accessible DIY |
| 3 blinks (pressure) | Partially | Check flue/drain yourself first; call if unclear |
| 4 blinks (high limit) | Yes — check filter first | If filter is fine, blower or limit switch issue |
| 5 blinks (flame w/o call) | No — call immediately | Gas valve issue — safety hazard |
| 7 blinks (low flame signal) | Yes — clean flame sensor | Easy fix with fine emery cloth |
| 8 blinks (ignitor circuit) | Yes — check/replace ignitor | Confirm with multimeter |
| 9 blinks (rollout) | No — call immediately | Combustion safety issue |
If Your Furnace Keeps Faulting After Repairs
Repeated faults despite repairs often indicate a failing control board, aging heat exchanger, or a gas valve issue. On furnaces over 15 years old, a recurring fault that requires a $300–$600 repair is often the signal to replace. A new Goodman factory-direct furnace with full warranty coverage will cost less over the next decade than chasing faults on aging equipment.
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