Most furnaces built in the last few decades have a small LED on the control board, visible through a window in the lower door. It blinks in patterns that tell a technician what the furnace has detected. You do not need to be a technician to read it. You just need the chart for your specific furnace, and it is usually taped to the inside of the door.
Important: codes differ by brand and model
There is no universal meaning for “three red flashes.” Carrier, Goodman, Lennox, Trane, Rheem, York and the rest all use their own patterns, and they change between model generations. Any website that gives you one list for every furnace is guessing. Always go by the label on your own unit.
Where to find your code chart
- The inside of the furnace door. Most manufacturers print the LED code table on a label on the blower compartment door or the control board cover.
- The installation or owner’s manual. Search the model number from the rating plate, usually found inside the cabinet, with the word “manual.”
- The manufacturer’s website using the full model and serial number.
How to read it without losing the code
- Read it before you open the door. On many furnaces, removing the blower door trips a safety switch that cuts power to the board and clears the code. Look through the sight window first.
- Count carefully. Many boards flash a number of times, pause, and repeat. Some use two groups, such as three flashes, pause, one flash. Watch at least two full cycles.
- Note the color. Some boards use different colors for different meanings.
- Write it down or record a short video on your phone. That is the single most useful thing you can give a technician.
What the common patterns tend to mean
Without replacing your own chart, these are broad patterns we see across many brands:
- Steady on or a slow, regular blink: often normal operation or standby. Many boards use a slow heartbeat blink to show they have power.
- Rapid flashing: on some models, a call for heat in progress, or a flame sense issue, depending on the brand.
- Off entirely: no power to the board. Check the breaker, the service switch next to the furnace, the door safety switch, and the 3 amp fuse on the board. See a furnace blowing its fuse.
- A counted fault code: points to a specific area such as the pressure switch, limit switch, ignition, flame sense or polarity. Your chart says which.
The faults behind the codes
Whatever the pattern, most furnace fault codes come back to a short list of causes:
- Pressure switch faults, often a blocked vent or a clogged condensate trap on high efficiency furnaces. See furnace leaking water.
- Limit switch faults, usually overheating from a clogged filter or restricted airflow.
- Ignition or flame sense failures, from a worn igniter or a dirty flame sensor. See furnace will not ignite.
- Rollout switch trips, which can indicate a blocked flue or heat exchanger problem and need a technician.
- Polarity or grounding faults, from wiring issues.
What you can do
- Replace the filter if it is dirty. It clears more limit faults than anything else.
- Make sure the vent pipes outside are clear.
- Reset power once at the service switch if the manual suggests it.
If the code returns, or it points to a rollout switch, gas valve or flame problem, stop resetting and call. A furnace that locks out repeatedly is protecting you from something. Make sure your carbon monoxide alarm works.
What we do
Send us the code and your model number when you call, and we can often bring the likely part on the first visit. We confirm the fault with meters rather than trusting the light alone, and we take a carbon monoxide reading on every gas furnace. Our diagnostic is a flat $69 and it is credited toward the repair. See furnace will not turn on and furnace repair in Sacramento.
Frequently asked questions
What does a blinking red light on my furnace mean?
It is a diagnostic code from the control board. The meaning depends on your brand and model, so check the code chart on the inside of the furnace door or in the manual.
Is a blinking green light on a furnace normal?
On many furnaces a steady or slowly blinking green or amber light indicates normal operation or standby. Confirm with the chart for your model.
Where is the furnace code chart?
Usually on a label inside the furnace door or on the control board cover. It is also in the installation manual for your model number.
Why did the furnace code disappear when I opened the door?
Removing the door often trips a safety switch that cuts power to the board and clears the code. Read the light through the sight window before opening the door.
Can I reset my furnace to clear the code?
Resetting power once is reasonable if the manual suggests it. If the code returns, the furnace has a real fault and needs to be checked.
Which furnace codes are serious?
Codes pointing to rollout switches, flame problems or gas valves should be checked right away. Make sure your carbon monoxide alarm is working.