Furnace won't start? Six checks before you call.

Every fall, the first cold night brings a wave of no-heat calls, and a good share are fixed in minutes without a technician. Try these first. If you smell gas, skip all of this: leave the house and call your gas utility from outside.

1. Check the thermostat

Make sure it's set to Heat, the set temperature is above room temperature, and the screen is on. Dead batteries are the most common "broken furnace" of all.

2. Check the furnace switch and breaker

There's usually a light-switch-style switch on or near the furnace. It gets bumped off during summer storage. Check the breaker panel too.

3. Look at the filter

A badly clogged filter can make the furnace overheat and lock itself out. Replace it, then turn the power off and on to reset.

4. Make sure the panel door is shut

The furnace won't run with its blower door off or loose; a safety switch stops it.

5. Check the outside vent pipes

High-efficiency furnaces vent through plastic pipes on an outside wall or the roof. Snow, leaves or a bird's nest can block them.

6. Count the blinking light

Most furnaces have a small window with a status light. The blink pattern matches a code on the panel label. Take a photo of it; it tells a technician a lot.

When to call

If none of that works, if the furnace starts and then shuts off after a few seconds, or if your carbon monoxide alarm sounds, it's time for a professional. Book furnace service, or call if it's below freezing.