Furnace Won't Turn On? 11 Checks to Run Before You Call

Work through the checks below in order. They start with the most common and easiest causes and move toward the ones you should only look at, not touch.

If you smell gas, stop here

A rotten-egg smell near the furnace is not a troubleshooting step. Leave the house, then call your gas supplier or 911 from outside or from a neighbor's home. Don't flip light switches, don't use a phone near the smell and don't try to light anything. Connecticut's utility regulator spells out what to do if you smell gas, and furnace makers print the same rules in their manuals. Rheem's user's manual for its gas furnaces says to call the gas supplier from a neighbor's phone and the fire department if you can't reach them.

The same goes for a carbon monoxide alarm. Carrier's guide to a furnace not turning on says to evacuate immediately if you smell gas or a CO detector sounds, and to call from a safe place. Don't go back in until the gas company says it's safe.

The checks, in order

1. Thermostat set to Heat and above room temperature

It sounds too simple, but it tops both Carrier's and Trane's lists. Trane's page on a furnace that won't start says to confirm the thermostat is on Heat and set above the room temperature. Some thermostats also need a switch moved or a button pressed before they call for heat.

Carrier suggests setting it at least 3 to 5 degrees above the current room temperature so there's no doubt the thermostat is asking for heat. If you have a programmable model, check the screen for an error code and look it up in the thermostat's manual.

2. Thermostat batteries and wiring

If your thermostat runs on batteries, replace them. On a Honeywell with a blank screen, our Honeywell thermostat reset guide walks through the checks. Trane lists this right after the setting check. On a hard-wired thermostat, Trane says to check for loose wires. Our thermostat wiring color guide shows which wire goes where if you need to take the faceplate off.

3. The furnace power switch

Most furnaces have their own on/off switch on the side of the unit or on a nearby wall. Bryant, a Carrier brand, notes in its furnace troubleshooting guide that it looks like a standard light switch. That makes it easy to flip off by mistake. Trane adds that it can also be mounted on a ceiling joist. Make sure it's in the On position.

4. The circuit breaker

Find the breaker labeled Furnace or Heat in your electrical panel. Carrier says a breaker stuck in the middle position has tripped: push it fully to Off, then back to On.

Reset it once. If it trips again, leave it off. Bryant and Trane both say a breaker that keeps tripping points to an electrical problem that needs a professional.

5. The blower door safety switch

On many furnaces, the panel over the blower compartment presses a small safety switch when it's closed. Goodman's service manual for its two-stage furnaces notes that this door interlock switch must be engaged for the furnace to get power. Trane's page on a blinking furnace light says some units cut both power and gas when the door comes off.

That matters most right after a filter change. Rheem's manual tells owners to put the doors back after changing filters, and its checklist for a furnace that fails to operate asks whether the blower doors are in place. Seat the panel firmly so it presses the switch. Never tape the switch down or run the furnace with the door off.

6. The air filter and the high-limit switch

A clogged filter starves the furnace of air. Carrier says the system can then overheat and shut itself down. The part that does the shutting down is the high-limit switch, which watches the temperature inside the furnace. Bryant says it should never be bypassed and that repeated limit shutdowns need a professional.

On some boards the limit can lock the furnace out for a while. Goodman's two-stage manual says that if the limit opens five times in a single call for heat, the furnace locks out for one hour.

Pull the filter and look at it. If it's gray and matted, replace it with the same size and type, with the airflow arrow pointing toward the furnace, as Carrier describes. Turn the furnace power off first. Rheem's manual calls for checking filters monthly during the heating season. Our guide on how often to change your furnace filter covers the different filter types. While you're at it, move furniture and drapes off the supply registers and return grilles, which Trane also lists.

7. The condensate float switch (high-efficiency furnaces)

High-efficiency furnaces, rated 90% AFUE (annual fuel utilization efficiency) or higher, make water as they run. Trane explains in its guide to a furnace leaking water that you can spot one by its PVC vent pipes, where older furnaces use metal. That water drains through a trap and a line, sometimes with a small pump.

If the drain clogs, Bryant says a float switch can stop the furnace from running to prevent water damage. Goodman's manual adds that the pressure switches also guard against a blocked condensate drain. Look around the base of the furnace and the drain line for standing water. If you find any, turn the furnace off. Bryant warns not to bypass a condensate safety switch or take internal parts apart, so this is a technician call. Trane says a technician should clear the drain line and pan and check the pump if you have one.

8. The gas supply

Carrier says to confirm that the gas valve to the furnace is open. Then check another gas appliance, such as the stove. If nothing gas-fired works, the supply to the house may be interrupted and you should call your utility.

Don't force or repair a gas valve. Bryant says never to attempt gas valve repairs yourself, and Rheem's manual warns that forcing a gas control can cause a fire or explosion. If you heat with oil rather than gas, our guide to an oil furnace that won't start covers the checks for that system.

9. The pilot light or electronic ignition

Newer furnaces don't have a standing pilot. They light the burners with an electronic igniter, and Rheem's manual is direct about it: do not try to light the burner by hand. Carrier says a failed ignition usually needs a technician.

Before you call, look at the LED on the furnace control board through the sight glass in the door. The flash pattern is a diagnostic code, and our Carrier, Goodman and Lennox code lists decode it, so you can tell the technician what the board reported.

Older furnaces with a pilot light are different. Trane says the homeowner can relight the pilot, following the instructions on the furnace label, but should call a professional if it won't stay lit. Our guide to the furnace pilot light covers relighting it.

10. The flame sensor

The flame sensor is a metal probe mounted at the burners that proves the gas has lit. Goodman's manual explains that if the control doesn't see flame, it shuts the gas valve and either retries or locks out. A nearly invisible coating can build up on the sensor over time and weaken its signal.

The usual sign is a furnace that starts, then shuts off, over and over. Bryant says those repeated start-then-shutoff cycles need a technician. Goodman marks sensor cleaning as a job for a qualified servicer only, and the sensor sits in the burner compartment, so note the pattern and call.

11. The error-code light on the control board

Most modern furnaces report faults with a blinking light on the control board. Trane notes that some furnaces show it from outside the cabinet; on others it sits behind the front panel, which you shouldn't remove while the furnace is running. Trane says a slow green blink usually means the furnace is idle, a fast green blink means it's heating, and a blinking red light generally means a fault. Write down the color and the blink pattern. Our Trane furnace error codes list decodes Trane's blinks and display codes.

Codes are not universal. Rheem's gas furnace technical FAQ lists one blink as a soft lockout on its boards, while some Goodman furnaces show letter and number codes on a digital display. Look up your code on the chart that came with your furnace or in its manual. Bryant notes that the model number is on the rating plate inside the cabinet.

Resetting a furnace that has locked out

After repeated failed tries to light, the control board stops trying. That's a lockout. Goodman's modulating furnace service manual sets the limit at three attempts in one call for heat, and Rheem's technical FAQ says its boards lock out after four. Both reset in similar ways.

• Turn the thermostat down so it stops calling for heat for 5 to 10 seconds, then set it back.
• Or switch the furnace power off, wait about 30 seconds as Carrier suggests, and turn it back on.
• Or wait: Goodman's control resets itself after a one-hour lockout.

Reset once. Goodman's manual notes that if whatever caused the lockout is still there, the furnace will lock out again. A second lockout means it's time for a technician, not more resets.

What you can fix and what needs a technician

If the furnace runs but only blows cold air, that's a different problem. Our guide to a furnace blowing cold air covers it.

When to call a technician

Rheem's manual says that if the blowers run and you've checked the thermostat, doors, power, breaker, gas and filter without finding the cause, there may be a serious problem with the installation. Trane draws the same line. Anything inside the burner compartment, any gas part and any safety switch that keeps tripping belongs to a licensed technician.

Before you call, write down the furnace's model number, the blink code if there is one, and what the furnace does when the thermostat calls for heat. That shortens the visit.