Most modern furnaces and many heat pump air handlers can tell you what's wrong. There's a small light on the control board that blinks in a pattern, and a chart that translates the pattern. Reading it before you call saves time, and sometimes it points to a simple fix like a clogged filter or a blocked vent.
Every brand uses its own codes. Rheem, Ruud, Goodman, Amana, Carrier, Bryant, Trane, American Standard and Lennox all print the chart for your exact model on the inside of the panel door. So the goal here isn't to memorize codes. It's to find the light, read it correctly and match it to your chart.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Don't turn the power off first
On many systems, cutting power clears the current code. Read the light before you reset anything.
Find the status light
Look for a small window or peephole in the lower panel of the furnace or air handler. Behind it is the control board with a red, green or amber light. If there's no window, you may need to remove the upper or lower door, but remember that most units shut off when the blower door comes off (that's the door safety switch).
Count the flashes carefully
Codes usually come as a number of flashes, a pause, then repeat. Some use two numbers, like 3 flashes, pause, 1 flash. Watch it two or three times through. The easiest way is to take a 20-second video with your phone so you can count later and show it to your technician.
Find the chart on the door
The code chart is usually a sticker on the inside of the blower door or the burner door. Take a photo of it too. If the sticker is missing or faded, the model number on the unit's rating plate lets a tech look it up.
Understand what the common codes mean
Codes vary by brand, but they usually fall into a few groups:
Steady on or slow normal blink: the system is powered and fine.
Pressure switch: the furnace can't prove the venting is clear. Check the outside vent pipes and the condensate drain hose.
Limit or overheating: usually airflow. Check the filter and the vents first.
Flame failure or ignition lockout: the burners didn't light or the flame wasn't sensed. Often a flame sensor, igniter or gas supply issue.
Rollout or flame rollout: flames came out where they shouldn't. This is a safety issue. Leave it off and call.
Reversed polarity or wiring: an electrical problem. Call a pro.Check the easy fixes the code points to
For limit codes, replace the filter and open the vents. For pressure switch codes, clear the outdoor vent pipes and check the drain hose for kinks or water. If the code clears and the system runs normally, keep an eye on it for the next day.
Tell your technician the code
When you call, give the brand, the code (or send the video), and what you've already checked. A tech who knows it's a pressure switch code on a Goodman furnace can show up with the right parts.
A few brand notes
Brands lay out their codes differently. Some Goodman and Amana furnaces flash a single number. Many Carrier and Bryant boards use two-digit codes (for example, a few flashes, a pause, then a few more). Rheem and Ruud boards often have a code chart printed right on the door with the flash count next to each fault. Trane and American Standard, and Lennox with its communicating systems, may show codes on the thermostat screen as well as the board.
Newer communicating and variable-speed systems may keep a history of recent faults, which is very helpful for problems that come and go. If your thermostat shows an alert or a service code, take a picture of that too.
Codes point a technician in the right direction, but they aren't the whole diagnosis. A pressure switch code, for example, can mean a blocked vent, a clogged drain, a failing inducer motor, a cracked hose or a bad switch. That's why I test the system rather than just replacing the part the code names. You only pay for the repair you actually need.
If you send us a short video of the blinking light and a photo of the chart when you call or text, we can often tell you whether it's something you can fix yourself, or come prepared with the right part.
When to call a pro
- Any rollout, flame rollout or gas valve code.
- Ignition or flame failure codes that come back after a reset.
- Wiring, polarity or control board codes.
- Limit or pressure switch codes that come back after you've changed the filter and cleared the vents.
- You can't find or read the code but the system won't run.
Still not working?
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Common questions
What does a blinking light on my furnace mean?
It's a diagnostic code from the control board. Count the flashes and match the pattern to the chart on the inside of the furnace door. A slow, steady blink often just means normal operation.
Where is the error code chart on my furnace?
It's usually a sticker on the inside of the blower or burner door. If it's missing, the model number on the rating plate lets a tech look up the chart.
Can I reset my furnace to clear the code?
You can turn the power off and on once after you've read the code. If the same code comes back, the problem is still there. Don't keep resetting it.
Do heat pumps have error codes too?
Many do, on the indoor air handler board, the outdoor unit's defrost board, or a communicating thermostat. Check all three.
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