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How do you read a furnace blinking light code?
Look through the small window on the lower furnace door, count the blinks in one group, then match that number to the chart on the door or inside the cabinet.
- Find the light. Most gas furnaces have a small LED on the control board, visible through a sight window in the lower blower door. It may be red, green, amber, or a combination.
- Watch before you touch anything. On many furnaces, opening the blower door trips a safety switch, cuts power to the board, and clears the code. Read the light with the door closed.
- Count one full group. A fault code is a set of blinks, a pause, then the same set again. Three blinks, pause, three blinks is code 3. Some boards use two digits, such as one long and three short blinks.
- Take a short video. Recording 20 to 30 seconds of the light with your phone avoids miscounting, and it gives the technician the exact pattern.
- Match it to the label. The code chart is usually printed on a sticker on the back of the blower door or inside the cabinet. Write down the code number and the wording next to it.
Codes differ between brands and even between models from the same brand, so a chart for another furnace can point you in the wrong direction. Always go by the label on your own unit.
What do common furnace light patterns mean?
The general pattern tells you whether you are looking at a normal status light or a fault, even before you check the label.
| What you see | What it usually means | What to do |
|---|---|---|
| No light at all | No power to the control board, or a blown low-voltage fuse | Check the breaker, service switch and blower door |
| Steady light or a slow, even blink | On many furnaces, power is on and there is no fault | If there is still no heat, check the thermostat |
| Blinks in groups with a pause | A fault code; the number of blinks is the code | Count one group and match it to the label |
| Fast, nonstop flashing | On some models a flame-sensing or wiring polarity fault; on others, normal operation | Look for "rapid flash" on the label |
| Two colors or two lights flashing | Some boards use one color for status and another for faults | Read each light against the label |
| The code returns right after a reset | The fault is still there | Stop resetting and call for repair |
What are the most common furnace fault codes?
The code numbers change from brand to brand, but most labels describe the same handful of problems. Here is what the wording usually points to.
Pressure switch open or stuck closed
The furnace could not confirm that the vent is drawing properly. Typical causes are a blocked vent pipe, a weak inducer motor, a cracked or wet pressure-switch hose, or a full condensate trap on a high-efficiency furnace. This code often goes with a furnace that tries to start and then stops; see why a furnace keeps shutting off.
Limit circuit open or high-limit trip
The furnace got too hot and shut the burners off. A clogged filter, closed vents or a struggling blower are the usual reasons. It is a common cause of a furnace that runs but pushes cool air; see why a furnace blows cold air.
Ignition failure or lockout
The furnace tried to light several times and gave up. A worn igniter, a dirty flame sensor, a closed gas valve or an empty propane tank are the usual suspects. If the furnace won't start at all, see what to check when a furnace won't turn on.
Flame sensed with no call for heat
The board sees a flame when there should not be one. This can mean the gas valve is not closing fully. Turn the furnace off at the thermostat and the service switch, and call. Do not keep running it.
Rollout switch open
Flames or heat came out of the burner area instead of going up the vent. A blocked flue or a problem inside the furnace can cause it. This is a safety shutdown, so leave the furnace off and have it inspected rather than resetting it.
Reversed polarity or poor ground
The board cannot sense the flame properly because the hot and neutral wires are swapped or the ground is weak. It often shows up after electrical work, a panel upgrade or a new outlet near the furnace.
Low-voltage fuse or 24-volt fault
A short in the thermostat wiring, a damaged wire or a failing component blew the small fuse on the control board. Replacing the fuse without finding the short usually means it blows again.
Codes after a power outage or a public safety power shutoff are common in North County. If the furnace starts normally after power returns and the code doesn't come back, the outage was likely the cause.
When should I call about a furnace light code?
Call right away for a rollout, flame-sensed or repeated limit code, and for any code that comes back after one reset.
- You smell gas or rotten eggs. Leave the house and call 911 from outside, then your gas company or propane supplier.
- The label says rollout, flame sensed with no call, or a gas valve fault.
- The same code returns after one reset.
- The low-voltage fuse blows again after it is replaced.
- You can't find or read the code label.
Got a code you can't clear? Give the technician the color and the blink count, and the repair visit starts from there.
Call Now +1 760 354 4278Does a fault code mean I need a new furnace?
No. A code points to one system, and most of them lead to a single-part repair: a switch, a sensor, an igniter, a fuse or a hose. A code becomes part of a replacement decision only when it traces back to a cracked heat exchanger or keeps returning on an older furnace that already needs major parts. The furnace replacement page covers how to weigh the two, and a yearly furnace cleaning helps keep sensors and drains from setting off codes in the first place.
Furnace blinking light: FAQs
Is a blinking green light on a furnace normal?
Often, yes. On many furnaces a steady green light or a slow, even green blink is a normal status signal that the board has power. A green or red light that blinks in counted groups with a pause in between is a fault code, so check it against the label inside the furnace door.
Will resetting the furnace clear the blinking code?
Turning the power off usually clears the light, but not the problem behind it. If the fault is still there, the same code comes back on the next start. Many control boards also store recent codes, so write the pattern down or take a short video before you reset anything.
What if I can't find the code label on my furnace?
Look for the model number on the rating plate inside the furnace cabinet. The manufacturer's installation or service manual for that model lists the codes. If you can't find either, note the color and the number of blinks and give that to the technician when you call.
Should I worry about a limit or rollout switch code?
Yes. Both are overheating or flame safety controls. A limit code often comes from restricted airflow, and a rollout code can point to a blocked flue or a problem inside the furnace. Do not keep resetting the furnace after either code; turn it off and call for repair.