A blinking light on a furnace control board is a status or fault code. The board is telling you that a safety circuit tripped or a component did not respond as expected, for example a limit, pressure switch, ignition or flame sensing fault. Note the pattern, look it up for your brand, and do only the safe checks before calling a pro.
Normal vs. fault blinking
Some boards blink a steady pattern during normal operation, and blink a numbered pattern when there is a fault. Your door label shows which is which. For example, on the Lennox ML180UH the LEDs use slow and fast flash combinations, while Carrier and Rheem use numbered codes.
Common fault families
| Fault family | What the board is seeing | Example codes |
|---|---|---|
| Limit / rollout | A temperature safety switch opened | Carrier 33, Goodman EE3, Trane E04, Rheem 33 |
| Pressure switch | Inducer draft not proven, or switch in wrong state | Goodman EE2, Carrier 31, Trane E3.2, Rheem 45 |
| Ignition / flame | Flame not sensed or lost | Carrier 34, Goodman EE0, Trane E2.1, Rheem 11 |
| Low flame signal | Flame sensor signal weak | Goodman EE6, Trane E08, Rheem 12 |
| Power / polarity / ground | Wiring or supply problem | Goodman EEA, Trane E06, Carrier rapid flashing |
| Lockout | Too many failed tries; furnace waits before retrying | Carrier 14, Rheem 10, Lennox alternating slow flash |
What you can check safely
- The air filter: the Goodman and Carrier manuals list blocked filters and restricted airflow among causes of limit faults.
- That supply and return registers are open.
- That the thermostat is set to heat and calling for heat.
- That outdoor intake and exhaust pipes (on high-efficiency furnaces) are not blocked by snow or debris. NFPA advises keeping furnace vents clear of snow build-up.
Anything inside the burner, gas valve, venting or wiring area is technician work.
Safety first: If you smell gas, don’t touch light switches or use a phone inside. Leave the building and call your gas utility from outside. Turn off power to the furnace before opening any panel, and never bypass or jumper a safety switch. Gas, venting, wiring and control-board work is for a licensed HVAC technician.
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Is a blinking furnace light dangerous?
The light itself is just a signal. Some faults point to safety issues such as flame rollout or venting problems, so look up the code and call a technician if the code is safety-related or keeps returning.
Should I reset the furnace when it blinks?
Write down the code first. A reset can clear some lockouts, but if the code comes back the cause is still there. See our guide on resetting a furnace.