Your furnace is blinking at you.
Here's exactly what it's saying.

Furnace control boards report faults as light patterns and display codes. Generic code tables on the internet are often wrong — different generations of the same brand use different maps. Every code here is tied to its exact series and verified against the manufacturer's service manual, with the source page cited.

Pick your furnace brand

Or decode the blink pattern directly

Watching a light flash and counting? Use the blink decoder — tell it what you see (three short, one long…) and it matches the code.

Why trust this over a random blog table?

1. Every code cites the service manual and page it came from — check our work.
2. Codes are bound to their exact model series. Same brand, different generation = different code map (we verified cases where the same flash count means opposite things).
3. We tell you when a fix is a $10 DIY part and when it's genuinely a technician's job — including the safety cases where you should stop and call.