Somewhere on your furnace — usually behind a small round window in the blower door, sometimes only visible with the door off — there is an LED on the control board. When the furnace faults, that LED flashes a pattern. Count the pattern and you have the code. It sounds foolproof. It goes wrong in four predictable ways.
Mistake one: cutting the power first
Most boards store the fault in memory, and most boards erase it when power cycles. Lennox’s newer control keeps its last 10 codes even through a power cut; Trane’s IFC replays its last 4 every time power comes back. Carrier’s manual is explicit that stored codes vanish when power is interrupted. So: look first, photograph the pattern with your phone if you can, and only then touch anything. A code you deleted is a diagnosis you pay a technician to redo from scratch.
How the counting works, brand by brand
Simple count (Goodman, Trane, Lennox, York)
The LED flashes a number of times, pauses — about 2 to 3 seconds — and repeats. Count the flashes between pauses. York’s manual is the most precise about it: flashes are a quarter-second on, a quarter-second off, groups separated by a two-second pause. Lennox’s newer control flashes fast — half a second on, half a second off — then a 3-second pause, which is why Lennox codes are so easy to overcount.
Two digits (Carrier, Bryant, Payne)
Carrier boards flash a two-digit number: short flashes for the first digit, long flashes for the second. Code 33 is three short, a pause, three long. If you count a Carrier pattern as one long string of blinks you will get a meaningless number like "6" — the single most common misread in furnace codes.
A display, not a light (Rheem, Ruud)
Newer Rheem furnaces have a small 7-segment display. Standby reads 0. Letters are modes, not faults: H heat, C cooling, F continuous fan. Faults are two-digit numbers, shown one digit at a time — and during a lockout the display alternates the lockout code (10) with the code that caused it. Read both.
The replay trap (Trane)
On power-up, Trane’s integrated control flashes its last four faults, newest first, with a 2-second gap between codes. Four faults can look like one very long code. If the count seems absurdly high, you are probably watching the replay, not a live fault.
Where the real chart lives
Inside the blower door — glued to it, or printed on the wiring diagram on its back — there is a fault-code chart for your exact board. Codes genuinely change between board generations: on this site there are two Goodman charts where 3 flashes mean a pressure switch stuck open on one board and closed on the other. When the door chart and any website disagree, the door wins. Every time.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
Once you have the code
Run it through the decoder, or go straight to your brand: Goodman, Carrier, Trane, Lennox, Rheem, York. Each table there names the manual it came from, so you can check our transcription against the same document.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.
Questions
My furnace light blinks continuously and never pauses. Is that a code?
Usually it is a state, not a count: a slow continuous blink is normal idle on Trane and York boards, a steady light means the Carrier control simply has power, and rapid flashing is its own listed condition (polarity on Carrier, twinning on Goodman and York). The tables for your brand list these states first for exactly this reason.
The light is off entirely. What does that mean?
Either the board has no power — switch, breaker, or a loose blower door holding its safety switch open — or, on a few controls (Goodman’s older board, Lennox SureLight), a dark LED is itself the listed sign of an internal control fault. Check power first; the brand tables say which boards treat darkness as a diagnosis.
Can I trust the code chart on a blog?
Only if it names its manual. Two of the charts on this site contradict each other inside the same brand, because the manufacturers changed boards. A chart that does not say which board it describes is a coin flip.