First, one habit that will save you money: write the pattern down — or take a photo of the light — before you touch the power switch. Cycling power clears the stored code on most boards, and a technician charges by the hour for diagnosis you just deleted.
The decoder
Three picks. If your combination is not in the lists, the honest answer is that this library does not cover your board yet — the chart inside your blower door does.
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.
Start from your brand
112 code entries across six brands, every one traced to a manufacturer document (the source is printed under each table — no blog telephone-game).
- Goodman & Amana furnace error codes — Two charts that disagree with each other — including a 3-blink code that means opposite things on different Goodman boards. (23 entries)
- Carrier, Bryant & Payne furnace error codes — Two-digit codes: short flashes, then long flashes. Home of code 33, the most-searched furnace code on the internet. (15 entries)
- Trane & American Standard furnace error codes — The slow blink that isn’t a fault, the fault replay that merges codes, and a 9-flash code with two meanings. (15 entries)
- Lennox furnace error codes — SureLight flash codes plus the newer numbered control with its Watchguard lockout mode. (25 entries)
- Rheem & Ruud furnace error codes — No blink counting — a little digital display that reads out two-digit codes, plus mode letters that look like faults. (19 entries)
- York furnace error codes — Red-flash codes from York’s own user manuals, including the manual-reset rollout warning. (15 entries)
Not sure how to even read the light?
Counting sounds trivial until the pattern has two digit groups, or the board replays four stored faults back to back. How to read furnace flash codes covers the sight glass, short vs. long flashes, Carrier's two-digit system, Rheem's display, and where the manufacturer's own chart is glued inside your furnace.
Furnace keeps shutting down and locking out instead of running? What "lockout" actually means — the 1-hour and 3-hour resets, by brand, and why a furnace that needs resetting every day is not fixed.
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 everybody asks
Is a blinking furnace light always a fault?
No. On Trane and York furnaces a slow blink is normal idle. On Carrier boards a steady light just means the control has power, and Rheem furnaces show letters (H, C, F) for normal running modes. The pattern matters more than the blinking.
Should I reset the furnace to clear the code?
Write the code down first — or photograph the light pattern. Cycling power clears stored codes on most boards, and on an intermittent fault you may not see that code again for a week. One reset after the basic checks (filter, thermostat, vents) is reasonable. Repeated resets are how small faults become big ones.
Why does the same blink count mean different things on different furnaces?
Because the code lives in the control board, not the brand badge. Goodman alone has two charts on this page where 3 flashes mean a pressure switch stuck open on one board and stuck closed on another. Model and board generation decide the meaning — which is why every table here names its manual.
Which furnace codes are emergencies?
Any code in the flame family — flame sensed with no call for heat, a gas valve fault, an open rollout switch — means turn the furnace off and call a technician. And if you smell gas, skip the codes entirely: leave the house and call the gas utility or 911 from outside.