Short answer
If a furnace lights then shuts off within seconds, a dirty flame sensor is the most likely cause. No ignition at all points to the igniter, pressure switch or gas supply. Read the blinking LED code through the panel window and give it to the tech.
Read the code
Most furnaces have an LED on the control board with a code chart on the door. Write it down before cycling power, because a reset can clear it.
Safe checks
Some causes need no tools.
- Filter clean
- Furnace power switch on
- Gas valve handle parallel to the pipe
- Condensate drain on 95% furnaces not frozen or clogged
When to stop
If you smell gas, leave the home and call your gas utility from outside. If the furnace keeps locking out, a safety device is doing its job; don't bypass it.
Questions
Can I clean the flame sensor myself?+
It's a common DIY task, but it involves opening the burner compartment. If you're not comfortable, let a tech do it during a tune-up.
Editorial note: this guide is general information for homeowners, not a diagnosis. Desertline Heating & Air is a referral service; a licensed contractor should inspect your equipment before repairs.