Most modern gas furnaces have a control board that reports faults with a blinking LED or a small digit display, seen through a window in the blower door. Count the flashes (or read the characters), then look the code up in the chart printed inside your furnace door or in its manual. The same number can mean different things on different brands, so always match the code to your brand and model.
Where to find the code
The status light or display is on the furnace control board, usually behind the lower (blower) door, often visible through a small sight window. Read and note the code before you turn the furnace off or reset it. Several boards also keep a fault history: the Goodman manual describes a list of the last 10 faults, the Trane Service Facts the last 6 faults, the Rheem manual the six most recent faults, and the Carrier manual a recall of the last stored status code.
How the brands show codes
| Brand (manual we used) | How the code is shown | Example |
|---|---|---|
| Carrier 59SC5 | LED flashes a two-digit status code: the first digit is the number of short flashes, the second the number of long flashes | Carrier 33: limit circuit fault |
| Goodman GR9S96/GD9S96 | 7-segment display shows a code such as EE3 | Goodman EE3: primary limit circuit open |
| Trane S9V2 | Seven-segment LED readout, e.g. E2.1 | Trane E2.1: retry exceeded |
| Lennox ML180UH | Red and green LEDs in combinations of on, off, slow and fast flash | Lennox red slow / green on: limit switch open |
| Rheem R96V | Dual 7-segment diagnostic display showing a numbered code | Rheem 11: failed ignition |
Step by step
- Look through the sight window without removing the door, if your furnace has one.
- Count the flashes, or write down exactly what the display shows. Note the pause between digits.
- Check the code chart on the inside of the door or in your manual.
- Look up your brand on our code lookup for the meaning, likely causes and safe checks.
Why the code matters
The code tells you which safety circuit or component the control board is unhappy with, for example a limit switch, pressure switch, flame sensing or ignition. That decides whether a simple check (like the air filter) is reasonable or whether you need a technician.
Safety first: If you smell gas, don’t touch light switches or use a phone inside. Leave the building and call your gas utility from outside. Turn off power to the furnace before opening any panel, and never bypass or jumper a safety switch. Gas, venting, wiring and control-board work is for a licensed HVAC technician.
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Does the same flash code mean the same thing on every brand?
No. Each manufacturer uses its own code list. Carrier 33, Rheem 33 and Goodman codes all mean different things, so use your brand's chart.
Will turning off power erase the code?
It can, so note the code first. Many boards also store recent faults: the Goodman, Trane, Rheem and Carrier manuals we used each describe a way to recall past fault codes.
Where is my furnace's code chart?
Usually on a label inside the blower door, and in the installation or service manual for your model.