About Furnace Fault Codes
What your furnace's blinking light or error code actually means
What this site is
A reference for furnace and HVAC fault codes across manufacturers. It exists because this information is real, in demand, and almost entirely locked inside PDF service manuals and scattered forum replies. The goal is one clear page per code: what it means, what causes it, what is safe to check, and where the line is between a homeowner fix and a technician call.
How the pages are written
Pages are drafted with AI assistance from manufacturer service literature, published installation manuals, and industry technical references, and every factual claim is checked against a listed source before publication. Fault code meanings are facts and are reproduced as facts; no manufacturer manual text is copied. Codes vary between manufacturers and even between models from the same manufacturer, so every page states which equipment it applies to. No page is published for a code whose meaning could not be verified.
The rule that shapes everything here
A furnace fault code is not a standard, and it is not standard within a brand either. Four of the eight manufacturers covered here publish more than one diagnostic code list, and the lists disagree with each other — on a Rheem, three blinks is an open limit switch on one and a pressure switch stuck closed on the other. That is why every page names the exact equipment its document covers, and why a code that appears on more than one list for the same brand carries a warning linking to the other meaning. The full account is here.
Coverage
- 259 codes documented, each stating the equipment or version its source covers
- 8 manufacturer groups, covering the brands they are sold under
- 45 specific model designations where the manufacturer publishes per model, resolving to 10 distinct code lists — boards sold under several names share one page
- 14 source documents, re-checked as reachable on every build. All but one are hosted by the manufacturer or its own content network; the exception is a document no manufacturer-hosted copy of could be found, cited from an archive and labelled as such wherever it is used
What this site does not cover, and why
Gaps are worth stating plainly. Each of these was pursued until the reason it could not be documented was clear, rather than quietly skipped.
- Nortek — Frigidaire, Maytag, Tappan, Gibson, Intertherm, Miller — the manufacturer literature site returns HTTP 200 with the body Unauthorized for every service document; only consumer brochures are public, and they carry no diagnostic tables. Two independent routes were tried.
- Daikin and Bosch — literature paths return 404, and Goodman’s own code database — which covers Goodman, Amana and Amana Distinctions — has no Daikin tables behind it either.
- Rheem and Ruud two-stage and modulating models — Rheem’s public flash-code page covers standard furnaces only and says so. The manuals for the rest are catalogued on Rheem’s own technical site, but every link points at a host that does not answer.
- ICP control-board codes — two ICP documents give contradictory meanings for identical flash counts because one describes the control board and the other a Honeywell SmartValve. The SmartValve set is published, scoped; the control-board set is held back until it can be sourced unambiguously.
None of these is covered from a third-party compilation. Fault code meanings are facts, but a compilation is not a manufacturer document, and nothing taken from one could be checked afterwards.
Corrections
If a page is wrong it should be fixed, and the fastest way to establish that is the sources: every page lists the document it was built from, with the model scope that document states, so the reasoning can be checked rather than taken on trust. Where a manufacturer's own documents disagree with each other, both are shown rather than one being chosen quietly.