A furnace that quits at 2 a.m. feels like an emergency, and sometimes it is. But most no-heat calls come down to a short list of causes that take a few minutes to rule out. Work through the checks below in order. If you smell gas at any step, stop, leave the home, and call 911.
The checks, in order
- Thermostat. Confirm it is set to Heat and the setpoint is above the room temperature. If the screen is blank or dim, replace the batteries. Carrier lists these as the first checks for a furnace that will not turn on.
- Furnace power switch. Most furnaces have a switch that looks like an ordinary light switch on or near the unit. It can get switched off by accident, especially after someone works near the furnace.
- Breaker. Check the furnace breaker in the panel. If it has tripped, flip it fully off, then on, once. If it trips again, leave it off and call a technician.
- Air filter. A clogged filter restricts airflow, and Carrier notes that it can cause the heat exchanger to overheat and trip a safety limit switch. Replace it if it is gray and packed with dust. See air filter sizes and MERV ratings to buy the right one.
- Gas shutoff valve. The valve near the furnace should have its handle parallel to the pipe when open. A valve turned crosswise is closed.
- Look at the control board light. Many furnaces show a blinking light through a small window. The flash pattern is a code that is specific to the model. Read the chart on the furnace door or in the manual, and write down the pattern before calling. Our guide to furnace blinking light codes explains how to read them.
What not to do
- Do not bypass safety switches. Limit and rollout switches shut the furnace down for a reason. Lennox warns against bypassing them, because they protect against overheating and fire.
- Do not keep resetting a breaker. One reset is fine. Repeated resets hide the real fault.
- Do not use a space heater near bedding or curtains as a long-term fix. Use it only as a short bridge until the furnace is repaired, and keep it clear of anything that burns.
If you smell gas
Leave the building immediately and call 911. Then report it to your gas utility. PG&E’s gas emergency number is 1-800-743-5000. Do not turn lights on or off, use your phone, or use a flashlight inside until the area is declared safe, because these can ignite gas. Do not try to relight a pilot light if you can smell gas. See furnace smells like burning or gas for more.
Common causes behind the checks
If the basics pass and the furnace still will not run, a technician will usually look at the following:
- The igniter or flame sensor, which the furnace uses to light and confirm the burner. See pilot light keeps going out.
- The condensate drain on a high-efficiency furnace. Trane says a professional should check the condensate drain line if the problem persists.
- A blown low-voltage fuse on the control board. See furnace blowing a fuse.
- A blower that runs but produces no heat. See furnace blowing cold air.
When to call for emergency service
Call right away if you smell gas, see soot, hear unusual noises, if a carbon monoxide alarm sounds, or if the furnace shuts off again within minutes of restarting. Heat loss is also urgent when a baby, an elderly person or someone ill is at home. If the furnace does not restart after the checks above, a technician can find the cause quickly.
We answer emergency calls 24/7, including weekends, across Roseville, Rocklin, Folsom and Elk Grove. See our emergency HVAC repair page, learn what repairs run in our furnace repair cost guide, or contact us. Our diagnostic is a flat $69, credited toward the repair.
Frequently asked questions
Why did my furnace stop working at night?
Common reasons are a thermostat setting or dead batteries, a tripped breaker or switched-off furnace power switch, a clogged filter that caused the furnace to shut down for safety, or a closed gas valve. Work through them in order before calling for service.
Is it safe to reset the furnace breaker?
Resetting a tripped breaker once is reasonable: flip it fully off, then on. If it trips again, stop. A breaker that keeps tripping means there is an electrical fault that needs a technician.
What does the blinking light on my furnace mean?
Furnaces flash a code on the control board, and the codes are specific to the model. Look at the label on the furnace door or the control board cover, or at the manual, for the code chart. There is no universal code list.
Should I relight the pilot light myself?
Only if you have an older furnace with a standing pilot and you know the procedure from the label. If you smell gas at any point, do not relight anything or operate switches. Leave the building and call 911, then your gas utility.
What number do I call if I smell gas?
Leave the area right away and call 911. Then report it to your gas utility. PG&E’s gas emergency number is 1-800-743-5000. Do not use light switches, phones or flashlights inside until the area is declared safe.