How to read your furnace's error code
- Takes about 5 minutes
- Beginner-friendly
- 1 tool needed
- Updated
- Reviewed by a licensed tech
Every modern furnace has an LED behind the door that is already telling you what is wrong. Counting its flashes takes five minutes and turns "it's broken" into a specific part.
Safety
You are reading the LED with the power ON and the panel off, which is the one time this compartment is live. Look, do not reach: line voltage sits on the board and the transformer, and the inducer can start without warning.
What you'll need
-
Phone camera, in video mode
Flash patterns repeat on a cycle and are easy to miscount. Film it and count on playback.
The fix, step by step
-
Find the sight glass and the LED behind it
Look at the lower door for a small round window, usually toward the middle. The status LED is behind it on the control board's edge. Most furnaces let you read it without removing anything — which is the point of the window.
-
Film the pattern for two full repeats
Codes are a repeating pattern of long and short flashes separated by a pause — "flash, flash, flash, pause" is a 3. Some boards use two colours, where red and green mean different fault families. Two full repeats on video and you will not miscount it.
-
Decode it against the chart on the door — not against the internet
The legend is printed on the inside of the door or the blower panel, and it is authoritative for YOUR furnace. Codes are not standard across brands: three flashes is a pressure switch fault on most Goodman boards and a limit circuit fault on others. The chart taped inside your cabinet outranks every chart online, including this one.
Safety — critical: If the code decodes to a flame ROLLOUT or a limit trip that keeps returning, stop. Those are the two codes that mean the heat exchanger or the flue is putting combustion products where they should not be, and they are a licensed tech's call, not a parts order.
-
Act on the family, not the flash count
Ignition-lockout codes point at the igniter or the flame sensor. Pressure-switch codes point at a blocked flue, a cracked hose or a failing inducer — in that order of likelihood, and note that the switch itself is the LEAST likely of the three. Limit codes point at airflow: change the filter first, every time.