Furnace Won't Turn On? 6 Things to Check

A furnace that won't start almost always traces to one of a few culprits. Work through the safe checks below — and stop immediately if you smell gas.

Safe-to-check items

• Thermostat. Set to Heat, temperature above room temp, and fresh batteries if it's battery-powered.
• Power. Check the furnace switch (it looks like a light switch near the unit) and the breaker.
• Air filter. A severely clogged filter can trip a safety limit and shut the furnace down to prevent overheating.
• Condensate drain. High-efficiency furnaces shut off if the condensate line is clogged and the float switch trips.

Leave these to a technician

• Ignitor or pilot failure — a cracked hot-surface ignitor is a common no-heat cause.
• Gas valve or limit-switch lockout — repeated lockouts protect you and signal a deeper fault.

Stop and call immediately if…

You smell gas (rotten egg), hear a hissing near the unit, or see soot. Leave the home and call your gas utility, then a licensed HVAC company. A cracked heat exchanger is a carbon-monoxide risk and is never a DIY repair.