The same code means different things: Five of the six biggest furnace brands publish more than one code list

A furnace fault code is not a standard. It is not even standard within one brand. Of the six largest North American manufacturers, five publish two or more diagnostic code lists whose meanings disagree — and almost nothing on the web says so.

Applies to: Every furnace brand covered on this site. Read this before trusting any flash count or error code you have looked up somewhere that did not ask which control you have.

What it means

The usual assumption is that a code identifies a fault, so looking it up gives an answer. What actually happens is that a manufacturer sells several control platforms concurrently, documents each in its own manual, and never publishes a cross-reference. Two furnaces with the same badge on the front can report the same signal and mean different things by it. This site was built one manufacturer at a time, and the pattern only became visible after checking all six deliberately: five had it. Carrier was the single exception, and that was confirmed by reading a second Carrier document in full rather than assumed. The reason this matters is not tidiness. On a Rheem, three blinks is an open limit switch on one list and a pressure switch stuck closed on the other — a filter problem versus a defeated safety interlock. On a Lennox, code 250 is an open limit circuit on one list and flame sensed out of sequence on the other — an airflow problem versus a gas valve concern. Acting on the wrong one sends you to the wrong part of the furnace, and in two of those four cases the wrong way round on urgency.

Most common causes

ICP — Heil, Tempstar, Comfortmaker, Arcoaire, KeepRite. Two documents give contradictory meanings for identical flash counts, because one describes the furnace control board and the other a Honeywell SV9541Q SmartValve. Five flashes is a high pressure switch fault on one and flame sensed out of sequence on the other.
Rheem — with Ruud and Weather King. The standard gas furnace flash code list disagrees with the sequence-of-operation blinks in the *RC 92+ and *RL 90+ installation instructions on three of the first four counts. Only two blinks means the same thing on both.
Lennox — communicating alert codes read from an S30 or S40 thermostat, against seven-segment E-codes read from the control board itself. Most numbers agree, which is exactly what makes 250 dangerous.
Trane and American Standard — flash counts on single-stage controls, alphanumeric seven-segment codes on two-stage variable speed models such as the S9V2. These share no numbering at all, so there is no overlapping number to misread — only the brand in common.
York, Luxaire and Coleman. Two control board generations, UTEC and last-generation White-Rodgers Emerson, documented in two tables inside a single service letter. A solid red LED is a generic control fault on one and an internal gas valve error on the other.
Carrier, Bryant and Payne — the exception. Two separate Carrier documents were read in full and both carry the same twelve status codes with the same meanings.
Goodman and Amana differ by model rather than by system: their published database is scoped per model, so six flashes is a rollout switch on a GMS and a rollout limit or blown control fuse on a GMSS9.

How to diagnose it

Find the model number on the rating plate inside the furnace cabinet before looking up any code. It is the single thing that resolves this, and it takes a minute.
Note where you read the code. A number on a thermostat screen and a number on a small display inside the furnace are frequently different systems, even on the same furnace.
Note the form of the code. A count of flashes, a two-digit status number, and an alphanumeric code like E2.1 belong to different tables — the shape tells you which.
On York, read the part number printed on the control board. The two generations are identified there directly.
If a source gives you a confident answer without asking which control you have, treat that answer as unverified — including any answer from an AI assistant, which will happily merge two manufacturers' tables into one.

How to fix it

Nothing to fix. This page exists so that a code is read against the right table.
Every code page on this site names the exact equipment its document covers, under "Applies to".
Where a code appears on more than one list for the same brand, the page says so and links to the other meaning.
When two readings are both plausible and the model cannot be established, act on the safer interpretation first — the one involving gas rather than airflow.

Notes

This is the clearest single reason to distrust a furnace code answer that does not state which control it applies to. It is also why every page here carries a model scope rather than a brand name alone: the brand is not enough information to answer the question.

Sources

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.