Safety Boundary
Stop before gas valve work, combustion adjustments, flue work, or any repair involving carbon monoxide risk.
Start Here
- Confirm the thermostat is calling for heat.
- Check filter condition and supply/return airflow.
- Make sure vents and exhaust terminations are not blocked.
- Count blink codes only after reading the furnace panel legend.
Decision Path
- No heat and no ignition: Thermostat call, door switch, power, and pressure-switch sequence come first.
Next: Review ignition and pressure codes. - Ignites then shuts down: Flame sensing, gas supply, and venting are professional-risk branches.
Next: Use flame and lockout code pages. - Short cycling: Airflow and overheating often show up before part failure.
Next: Compare limit and airflow codes.
Stop and Call for Service
- Gas smell
- Carbon monoxide alarm
- Soot, flame rollout, or melted wiring
- Repeated ignition lockout
Next Code Guides
Once the symptom branch is clear, compare the matching brand-specific pages in the Furnace error-code library.