Updated 2026-06-25

How We Research Furnace Codes

Where the code meanings come from, what the confidence flags mean, and why you should always confirm with your furnace's manual.

Sources

Each brand's flash/fault codes are researched from that manufacturer's own installation and service manuals, which publish the diagnostic code table, cross-checked against established HVAC technical references. The sources we used are listed at the bottom of every page.

Confidence flags

• high — the code meaning is stated directly in the manufacturer's published manual.
• medium — the meaning is well documented but can vary by furnace model or board revision; check your specific model's manual.

Safety

This site is written for homeowners doing basic, non-invasive checks (filters, vents, thermostat, resetting power). Anything involving the gas valve, burners, flame sensor, or wiring is for a licensed HVAC technician. We deliberately do not publish steps that bypass safety switches.

Always confirm against your furnace's own manual and, when in doubt, a licensed technician. Spotted something out of date? Email [email protected].