Heating
Furnace Error Codes: What Those Blinking Lights Mean
February 1, 2025
7 min read
Your furnace communicates problems through blinking LED codes. Here's what the most common codes mean and what to do.
By Energy Solutions — Family-run in Antioch since 1998
Most modern gas furnaces have a diagnostic LED behind the lower access panel that blinks a pattern when the furnace faults out. Manufacturers use different codes — the exact meaning is on the wiring diagram inside the panel — but the common categories are the same.
The Common Codes
If you've noticed any of these in the last two weeks, don't wait it out:
2 flashes — Pressure switch stuck open. Usually a blocked flue, condensate line, or bad inducer motor.
3 flashes — Draft/pressure issue. Check the vent for bird nests and the condensate trap for water.
4 flashes — Open limit switch. Overheating. Usually a clogged filter or blocked return.
5 flashes — Flame sensor fault. Dirty flame sensor. Cheap fix, often a first-call diagnosis.
6 flashes — Rollout switch. Serious. Cracked heat exchanger or blocked flue. Shut it off and call.
7 flashes — Gas valve fault. Usually the valve or ignitor.
Steady on / no flash — Control board. Board failure or no 24V power.
Write down the code before you call. It cuts diagnostic time in half.
Frequently Asked Questions
Where is the LED code light on my furnace?
Look through the small sight glass on the lower access panel. You'll see a red or green LED blinking a pattern. Count the flashes between pauses.
Is a flashing furnace light dangerous?
Not by itself. It's the furnace telling you why it shut down. Codes involving flame rollout, pressure switches, or limit switches can indicate a safety issue — don't override them.
Can I reset my furnace myself?
Yes — turn the furnace switch off for 60 seconds, then back on. If the same code returns, call a technician. Don't keep resetting a furnace that keeps locking out.
Free In-Home Estimates — Even in July
Family-run HVAC in Antioch since 1998. EPA-certified technicians. BBB-accredited. Authorized Bryant dealer. 10-year manufacturer warranty on every install. We'll come out, look at your system, and tell you straight what's going on — repair, service, or replacement. No pressure, no upsell.
CA Lic. #1132058
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