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.
How a modern furnace starts
Understanding the sequence helps you describe what you see. The thermostat calls for heat, the draft inducer motor starts, the pressure switch confirms venting, the hot surface igniter glows, the gas valve opens, the burners light, and the flame sensor proves the flame. The blower comes on a short time later.
- Inducer runs but no glow: igniter or pressure switch
- Glow but no flame: gas supply or gas valve
- Flame for a few seconds then off: flame sensor or grounding
Short cycling versus lockout
A furnace that starts and stops repeatedly every few minutes may be overheating from restricted airflow, often a dirty filter or closed registers. A furnace that tries three or so times and then stops entirely has gone into lockout; many reset after about an hour or after a power cycle.
High-efficiency furnace issues
Condensing furnaces (about 90% AFUE and above) use PVC vent pipes and a condensate drain. A blocked intake or exhaust pipe, often from snow, ice or debris, or a clogged condensate trap can stop ignition.
- Check outdoor PVC pipe terminations for blockage
- Look for water in the furnace cabinet
- Note any gurgling from the condensate trap
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.
Why does my furnace click but not light?+
Clicking usually means the control board is trying to start the sequence. A cracked igniter, a stuck pressure switch or no gas supply are the common causes.
How long should a furnace igniter last?+
Hot surface igniters often last several years but are fragile. Touching the element can shorten its life, so handle replacements by the base only.
Is it safe to keep resetting my furnace?+
One or two resets to read a code is fine. Repeated lockouts mean a safety device is tripping, so call a technician rather than continuing to reset it.
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.