Furnace Not Turning On? Here's What to Check Before Calling for Repair

A furnace that won't turn on is one of the most unsettling things to discover on a cold night, but before assuming the worst, a handful of simple checks catch a surprising number of these calls, and most of them take less than five minutes.

Start with the thermostat. Make sure it's set to Heat, not Cool or Off, and that the set temperature is actually above the current room temperature. If the display is blank or dim, check the batteries — a dead thermostat battery is one of the most common reasons a furnace never gets the signal to start in the first place.

Next, check power at the source. Look for a tripped breaker in your electrical panel, and find the furnace's own power switch — it looks like an ordinary wall light switch, usually mounted on or near the unit itself, and it's easy for it to get bumped off during cleaning or other work nearby.

If power and the thermostat both check out, look at the filter. A filter that's gone too long without being changed can restrict airflow enough to trip a safety switch that shuts the whole system down before it overheats. Our guide on how often to change your filter covers how to tell if yours is overdue.

Some furnaces have a reset option on the control board behind the access panel, and a simple power cycle — off for about a minute, then back on — resolves more no-start issues than people expect. If your unit instead shows a blinking light through a small window on the panel, that code usually points directly to the fault; Carrier's furnace troubleshooting guide has a rundown of what the common blink patterns mean across most gas furnace brands.

If you've checked all of this and the furnace still won't start — or if you ever smell gas or hear a hissing sound near the unit — stop troubleshooting and call for help. Ignition, flame sensor, and gas valve issues aren't DIY territory, and a gas smell specifically means leaving the house and calling your gas utility before doing anything else.

We've also put together a broader look at what tends to go wrong with furnace and heat pump repairs here in Hemet once you're past the basic checks. And if you'd rather catch these issues before they turn into a no-heat night, our comfort packages include a seasonal tune-up built for exactly that.