Check the Air Filter
A badly clogged air filter can cause the furnace to overheat and shut off, so replace it if it's dirty. It can be hard to tell with some filters, so consider installing an air filter gauge that lets you know when it's time to replace the filter.
Check the Power
Make sure the power is on! The power switch for your furnace looks like a regular light switch and can get bumped and turned off accidentally. If the switch is off, just flip it back on. See more simple furnace fixes you can DIY.
Check the Thermostat
Be sure your thermostat is set to 'heat' and at a temperature higher than the temperature inside the house. Thermostat still not working? Here's how to adjust it.
Check the Gasline Shutoff
Check the ball valve on the pipe that supplies gas to your furnace and make sure it's open all the way. (Here's how to find the gas shutoff valve.) When the valve's handle is parallel to the pipe, it's open.
Check the Door Switch
Check the door switch. Whenever you remove the access door on the furnace, a little safety switch shuts everything off. Sometimes this switch will stay turned off if the door isn't completely closed. Plus: Do you need a new furnace? You might not.
Check the Intake and Exhaust Pipes
Newer high-efficiency furnaces will shut off if something like a bird or ice buildup blocks either the fresh-air pipe or the exhaust pipe. You'll need to go outside and peek inside the pipes to see. Sometimes a critter can get lodged in the pipe all the way back to the furnace and you won't be able to see it without taking the pipe apart, a job best left to a pro.
Check the Condensate Line
High-efficiency furnaces produce water called 'condensate.' If the drain tubing for the condensate gets clogged, the furnace shuts off. Check the tubing and clear any clogs. Or, better yet, just replace the tubing.
Check the Cold-Air Returns
If you have some rooms that are warmer or colder than others, the problem might be a blocked air return. They're usually located in the floor or low on an interior wall. When blocked, cold room air can't 'return' to the furnace to be reheated.
Get a Carbon Monoxide Detector
When heat exchangers crack or there's a leak in the exhaust pipe, deadly carbon monoxide can seep into your home's living space. That's why it's critical that you have a working carbon monoxide detector. You can buy two-in-one carbon monoxide and smoke detectors, or stand-alone units that you just plug into a wall (with backup batteries) for $20 to $40 at home centers.
Adjust the Dampers
Check dampers to make sure they're not restricting airflow to the rooms that need it. Some need to be adjusted differently for winter and summer. You'll usually find them near the supply-air plenum on the large ducts that feed the rooms in your house. Here's how to fix a noisy vent hood damper.
The Best Furnace Advice: Change the Filter
Clogged air filters are usually to blame when a furnace stops working. Dirty air filters are also hard on blower motors and heat exchangers, so it's a good idea to replace your air filter before there's a problem. It can be hard to remember to check it regularly. That's where an air filter gauge can help; it eliminates guesswork by measuring airflow and telling you exactly when it's time to change your air filter.
What 'Efficiency' Means
An AFUE (annual fuel utilization efficiency) rating on a furnace tells you how efficiently the furnace turns fossil fuel into usable heat. An older-style furnace will have an efficiency rating of 56 to 83 percent, while a modern 'high-efficiency' furnace will have a rating of 90 percent or higher. That means 90 percent of the furnace's fuel becomes usable heat for your home, while the other 10 percent is lost with the exhaust up the chimney.