Furnace Not Turning On: What to Check, From the Thermostat to the Gas Supply

A furnace that does nothing when the thermostat calls for heat, no burner and no blower, is the most urgent furnace problem and often the easiest to fix. A gas furnace still needs electricity for its controls, fans, and igniter, and it refuses to start whenever a switch or safety sensor says something is wrong. Many no-heat calls end at a thermostat setting, a switch that looks like a light switch, or a panel that was not pushed closed.

This page lists the causes in the order to check them, which ones you can fix without opening the burner compartment, and where the technician takes over. If the furnace runs but the air stays cold, see furnace blowing cold air. If it starts and stops every few minutes, see short cycling.

First, Watch What It Does

Set the thermostat to HEAT a few degrees above the room temperature and stand by the furnace for a few minutes. A gas furnace starts in order: a small draft fan (the inducer) runs, the igniter glows orange or clicks, the burners light, and the main blower follows shortly after. Where the sequence stops tells you where to look.

• Nothing at all: no sound, no fan. Start with causes one through three.
• The small fan runs but nothing lights, or the burners light and go out within seconds. After a few tries, most furnaces lock out. See causes four through seven.
• The burners light but the air stays cool, or it cycles every few minutes: see the pages above.

Most Likely Causes

1. The thermostat. Confirm the mode is HEAT and the setpoint is above the room temperature; Carrier suggests three to five degrees above. Replace the batteries; Lennox notes weak ones cause trouble before they are dead. On a programmable or smart thermostat, look for a setback schedule or a hold keeping a low temperature. A blank smart thermostat is a clue: ecobee notes its thermostats run on power from the heating equipment, so a dark screen can mean the furnace has lost power. One that reboots, drops offline, or keeps losing power may need a C-wire; our C-wire guide covers the safe check. A quick test: set the fan to ON. If the blower runs, the thermostat and furnace power are working and the problem is on the heating side. Set it back to AUTO afterward.
2. Power: the service switch and the breaker. The service switch looks like a light switch and sits on the side of the furnace, on a nearby wall, or on a ceiling joist, which makes it easy to flip off by mistake. Make sure it is on. Then check the breaker labeled furnace or heat. A tripped breaker sits between on and off; push it fully off, then on. Reset it once. If it trips again, leave it off and call a pro; our breaker page explains why a second trip means stop.
3. The blower door is loose or off. Many furnaces have a safety switch behind the lower panel, the blower door, that cuts power when the panel is removed. If the panel is not seated squarely, often right after a filter change, the furnace will not run at all. Turn off the service switch, reseat the panel so it presses the switch, fasten it, and turn the power back on. Never tape the switch down or run the furnace with the panel off.
4. A clogged filter, a high-limit trip, or a lockout. A packed filter starves the furnace of air until the high-limit switch shuts the burners off to protect the heat exchanger. The control board also locks out after repeated failed ignitions (four tries on many Rheem models). Before you touch the power, find the status light, usually visible through a window in the blower door, and write down its pattern, such as three blinks and a pause. The legend is usually on a label inside the door. Cutting power can clear the code, so read it first. Then change the filter, arrow toward the furnace, per our filter guide, and reset once: service switch off, wait about a minute, back on. If it locks out again, stop and call.

5. The gas supply. If the igniter glows or clicks but the burners never light, check whether anything else on gas works: does a stove burner light, is the water heater making hot water? If nothing on gas works, call the gas utility. On propane, the tank may be empty; propane safety guidance is that your retailer or a qualified technician must leak-check the system before the gas is turned back on, so do not open the tank valve yourself. If only the furnace is affected, look at, but do not turn, the shutoff on the gas line to the furnace: a lever in line with the pipe is open, across it is closed. If it is closed and you do not know why, leave it and call the technician.
6. A blocked vent pipe or a full condensate pump on a high-efficiency furnace. Condensing furnaces have two white plastic pipes through the wall or roof for combustion air and exhaust. Snow, ice, or a nest over the ends keeps the furnace from proving a clear vent, and it will not start; Trane advises checking the pipes whenever the furnace quits after heavy snow. Clear the pipe ends by hand, from outside. These furnaces also make water, and if the drain clogs or the condensate pump fills, a float switch keeps the furnace off to prevent water damage. Check for water around the pump and make sure it is plugged in; then unplug it before you check that its float moves freely. Clearing the trap is the technician's job.
7. Ignition: the igniter, the pilot, or the flame sensor. Most newer furnaces light with a hot surface igniter, which glows, or a spark igniter, which clicks. An igniter that never glows is often cracked or failed; one that glows or clicks without lighting the burners points to the gas supply or the gas valve. Burners that light and go out within seconds, again and again, usually mean a dirty flame sensor; our short cycling page explains it. On an older furnace with a standing pilot, the relighting steps are on the furnace's label; relight only if you do not smell gas, and follow them exactly. If you smell gas at any point, leave the house and call from outside. If the pilot will not stay lit, stop and call. All of these parts sit in the burner compartment, and that belongs to the technician. So does a tripped rollout switch on the burner box: it means flame left the burner area.
8. The furnace is at the end of its life. Lennox and Trane put the typical life of a gas furnace at 15 to 20 years, and the Department of Energy suggests weighing new equipment once a system passes 15. If yours is that old and has needed several repairs lately, ask for a replacement estimate along with the repair quote.

What You Can Fix Yourself

• Set the thermostat to HEAT above room temperature, replace its batteries, and clear any hold.
• Turn the service switch on, and reset a tripped breaker once.
• Reseat the blower door so it presses its safety switch.
• Record the status light code, change the filter, and reset the furnace once.
• Check the stove and water heater, and call the gas utility or propane supplier if nothing on gas works.
• Clear snow and debris from the outside pipe ends, and check the condensate pump.

When to Call a Pro

Call a licensed HVAC technician when the breaker trips a second time, the furnace locks out again after one reset, the burners will not light, a pilot will not stay lit, the gas line valve is closed and you do not know why, a rollout switch has tripped, or the condensate trap is clogged. Tell them whether the small fan ran, whether the igniter glowed or clicked, whether the burners lit, and the blink code; it shortens the visit. If a CO alarm sounds, get everyone outside and call 911 from there. Do not go back in to check the furnace.

While you wait, an electric space heater can keep one room comfortable. Keep it 3 feet from anything that can burn, plug it straight into a wall outlet, and turn it off when you leave the room or go to sleep; our space heater guide covers safe models. Never heat the house with an oven or stovetop, or bring a fuel-burning heater, grill, or generator inside.

Prevent It

Book a tune-up every fall, as ENERGY STAR and the Department of Energy recommend; the technician cleans the flame sensor and checks the igniter, gas pressure, heat exchanger, and venting. Check the filter monthly and change it every one to three months, keep registers and returns clear, and clear the outside pipe ends after every snowfall. Replace thermostat batteries once a year. When you travel in winter, keep the heat no lower than 55°F and have someone check the house, so a furnace that quits does not become frozen pipes. And keep a CO alarm on every level of the home and outside sleeping areas, as the CPSC recommends, and test them monthly.

Frequently Asked Questions

Why won't my furnace turn on when the thermostat says heat?

Check that the setpoint is a few degrees above the room temperature, then the service switch, the breaker, and the blower door. If all of that checks out, read the status light; the furnace is probably locked out on a limit, ignition, or vent fault.

How do I reset my furnace?

Record the status light code first. Then turn off the service switch, wait about a minute, and turn it back on, once. If it locks out again, the fault is still there, and repeated resets only delay the diagnosis. On a gas furnace, do not press a reset button on the burner box; that is a rollout switch, and a trip means the technician should look. On an oil furnace, the red button on the burner's control box is the reset: press it once, and if the burner does not start or locks out again, call your oil company or technician. Honeywell's oil burner controls stop accepting resets after three lockouts in a row, to prevent repeated resetting.

Why does my furnace click but not turn on?

Clicking usually means the call for heat is getting through and the furnace is trying to light, but ignition is failing. Check the gas supply first. If gas is available and it still will not light, the igniter, gas valve, or flame sensor is next, and those are the technician's.

The Bottom Line

Thermostat, service switch, breaker, blower door. Then read the status light, change the filter, and reset once. Check the other gas appliances and the pipes outside. If the breaker trips again, the furnace locks out again, or the burners will not light, call the technician. If you smell gas, leave first and do not troubleshoot.

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