Short answer
When a gas furnace won't turn on, check that the thermostat is calling for heat, the furnace switch and breaker are on, the front door is seated, the filter isn't clogged and the outdoor PVC vent pipes aren't blocked by snow. Then read the blinking light code and call a technician with it.
Quick checks
Most no-heat calls in the first cold week of the year have a simple cause.
- Thermostat set to Heat, setpoint above room temperature, fresh batteries.
- The light-switch-style furnace switch near the unit is on.
- The breaker is on.
- The blower door is fully closed; a safety switch keeps the furnace off if it isn't.
- The filter is clean.
Outdoor vent pipes
High-efficiency furnaces have two white PVC pipes on an outside wall or the roof. Drifting snow and ice can block them after a Kansas City storm. Clear them gently, then reset the furnace by turning the switch off for 30 seconds and back on.
Read the error code
Most furnaces have a small window where an LED blinks a code. The code chart is usually on the inside of the door. Write it down; it tells the technician whether it's an ignition, pressure switch or limit problem before they arrive.
Common failed parts
A dirty flame sensor makes the burners light, then shut off within seconds. A cracked hot-surface ignitor means clicking with no glow. A clogged condensate trap trips the pressure switch. These are routine repairs for a technician.
When to call a technician
- The reset doesn't help or the furnace locks out again.
- You smell gas or a CO alarm sounds: leave first and call the gas utility.
- Temperatures are below freezing and the house is cooling fast.
What the technician does: see furnace repair.
Common questions
Can I clean the flame sensor myself?
Many handy homeowners do with fine steel wool, but it means removing a part near the gas burners. If you aren't comfortable, have a technician do it.
Is it safe to keep resetting my furnace?
A reset or two is fine. Repeated lockouts mean a safety device is doing its job, and the cause needs diagnosis.
Editorial note: this guide is general information for Kansas City-area homeowners, not a diagnosis. Conditions vary by home, so a licensed HVAC technician should confirm the cause before any repair.