Furnace not turning on? Here is how to find the cause

First, which problem do you have?

A furnace that "will not turn on" actually breaks three different ways, and knowing which one you have points you at the right checks:

• Totally dead. No display on the thermostat, no hum, no lights inside the furnace. This is almost always power: thermostat batteries, a breaker, the power switch, or the access panel. Start at the top of the list below.
• Powers on but will not light. You hear the furnace try, maybe a click or a fan, but no heat. This points to the filter, the gas supply, or the ignition parts further down.
• Runs but blows cold. The fan moves air but it is not warm. That is a different symptom with its own causes, covered on the furnace blowing cold air page.

Check the thermostat

The thermostat tells the furnace when to run, so start there. Confirm three things:

• Set to HEAT, not OFF or COOL.
• Target temperature is above the room temperature. If the room reads 70 and you set 68, the furnace has no reason to fire. Push it up five degrees and listen.
• Batteries are good. A blank or dim screen on a battery model is a dead giveaway. Fresh batteries cost a couple of dollars and fix a surprising number of no-heat calls.

Also cancel any "hold" or vacation program that might be overriding your setting, and set the fan to AUTO. If the screen is completely blank and new batteries do not wake it, the thermostat itself or its wiring may be the issue, which the thermostat not working page walks through.

Check the breaker and fuse

Gas and propane furnaces still run on electricity, so a tripped breaker leaves them dead. Open your electrical panel and look for the furnace breaker. A tripped breaker often sits in the middle, not fully on or off. Flip it firmly all the way OFF, then back ON.

One important safety point: if the breaker trips again the moment you reset it, stop. Do not keep flipping it back on. A breaker that trips immediately is telling you something is shorted or failing, and repeatedly resetting it is a fire risk. That one is a service call.

Find the furnace power switch

This is the fix people feel silly about, and it is one of the most common. Somewhere on the furnace or on the wall right next to it is a switch that looks exactly like an ordinary light switch. It is often unlabeled, sometimes at the top of the basement stairs, and it gets flipped off by accident all the time, by a cleaner, a kid, or you reaching for a light in a dark basement.

Find it, switch it ON, and give the furnace a minute to start its cycle. If a furnace went dead for no obvious reason, check this before anything that costs money.

That power switch is also how you reset a furnace. There is no special reset button on most units. To restart one that locked itself out, flip the power switch off, wait about thirty seconds, and flip it back on, which clears the control board and lets the furnace run through its startup sequence again. If it fires and then quits the same way a minute later, it is not a glitch, it is the furnace catching a real fault, and the rest of this page points you at which one.

Make sure the front panel is fully closed

Your furnace has a removable front panel that covers the blower. Behind it is a small push-button safety switch, called a door interlock, that only lets the furnace run when the panel is pushed in all the way. If the panel is off, loose, or sitting crooked, that button stays out and the furnace will not start at all.

This one trips people up right after they change a filter or poke around the unit. Push the panel firmly until it is flush and the latches or screws are snug. A warped or misaligned panel can fail to press the button even when it looks shut. This switch is there to keep the blower from pulling exhaust gases into your home, so never tape it down or jam it to bypass it.

Change a clogged air filter

A filter packed with dust starves the furnace of airflow and makes the heat exchanger overheat. When that happens, a safety switch shuts the burners off to protect the unit, which can look like a furnace that will not stay on or will not start. Pull the filter and hold it up to a light. If you cannot see light through it, replace it with the same size, arrow pointing toward the furnace.

Let the furnace cool for a bit after a fresh filter and try again. One caution: if the furnace keeps shutting itself off from overheating, do not just keep restarting it. Fix the airflow first. Our guide on how often to change a furnace filter covers the schedule and the warning signs.

Check the drain on a high-efficiency furnace

If your furnace has white PVC pipe coming out of it instead of a metal flue, it is a high-efficiency condensing furnace, and it makes several gallons of water a day that drain away. When that drain line clogs with gunk, the water backs up and a small float switch rises and shuts the furnace off to stop an overflow. The furnace looks dead, but the real problem is standing water.

Look for water around the base of the furnace or a full drain pan. You can often empty the pan and flush the drain line with a mix of warm water and white vinegar to break up the clog. If you cannot find the float switch or the line will not clear, a tech can do it on a basic service call, usually $100 to $200.

Check the gas (look, do not repair)

No gas means no flame. You can check the supply, you just do not repair anything gas-related yourself:

• The gas valve at the furnace should have its handle running parallel to the pipe, which means open. Sideways across the pipe means closed.
• Propane homes: check the tank gauge. An empty tank is a common no-heat cause in winter.
• Other gas appliances: if your stove and water heater are also out, the problem is the gas supply or the utility, not your furnace.

Again, if you smell gas at any point, do not flip switches or hunt for the cause. Leave and call your gas utility from outside.

Older pilot light or newer igniter

How your furnace lights the gas depends on its age. An older furnace has a standing pilot flame that can blow out, and you can relight it by following the printed instructions on the unit itself. A newer furnace lights with an electronic igniter that glows or sparks, and if that part fails, the gas never lights. You cannot fix an igniter yourself, but it is an inexpensive part for a tech to swap, covered in the cost list below.

If it tries to start but will not light

A furnace that has power and is clearly trying, but never produces heat, is telling you which part failed by how it behaves. Listen and watch through one full attempt, then match what you saw:

• It clicks and the small fan runs, but no flame, then it clicks again and retries. The igniter is most likely cracked or worn out, so the gas never lights. This is one of the most common no-heat repairs and an inexpensive part for a tech.
• It lights, runs for a few seconds, then shuts the flame off and tries again. This is the classic dirty flame-sensor signature. The sensor cannot confirm the flame, so the board cuts the gas as a safety. Cleaning or replacing it is the cheapest real fix on the list below.
• You hear nothing try to start at all, no fan, no click. Go back to the power-side checks above, because the furnace is not getting a run signal. The thermostat, a safety switch, or the control board is stopping it before ignition.
• It runs, heats, then quits after a few minutes and the fan keeps going. That is an overheating shutdown, usually a dirty filter or blocked vents starving the airflow. Fix the airflow rather than restarting it repeatedly.

None of these are gas-side repairs you do yourself, but knowing the pattern tells you whether you are looking at a cheap fix or an expensive one before the tech arrives, and it cuts time off the diagnosis if you describe what you saw.

Repairs that need a tech, and what each one runs

If the free checks above did not bring it back, you are into parts that a technician replaces. These are installed prices, parts and labor together, so you know whether you are looking at a cheap fix or a big one before the truck shows up. Regional labor rates move these around.

If a tech quotes a repair over about $500 on a furnace that is fifteen years or older, it is worth pricing a replacement before you commit, since you may be paying a third of a new furnace to patch an old one. Our guide on when to replace a furnace walks through the warning signs and the repair-versus-replace math, the gas furnace cost guide covers pricing, and you can get a free quote from a local installer to compare against the repair bill.

When to stop and call

Some signs mean you stop troubleshooting and pick up the phone:

• You smell gas. Leave the house and call the gas utility from outside. Do not flip switches.
• The breaker trips again right after you reset it. That is a short or a failing part, not a nuisance trip.
• A carbon monoxide alarm sounds. Get everyone out and call for help.
• You see water pooling, burn marks, or smell something scorching around the unit.
• The furnace keeps shutting itself off after you cleared the filter and the obvious stuff.

None of those are worth the risk of a DIY save. Shut the furnace off at its power switch and call a licensed tech. If you do not already have a carbon monoxide detector on each floor with a bedroom, add one. They cost about $25 and are required by code in most states for any home with a gas appliance.

Common questions about a furnace that will not start

What is the first thing to check when the furnace will not turn on?

The thermostat. Confirm it is set to HEAT, the target temperature is above the room temperature, and the batteries are fresh if it uses them. A blank screen or a setting that does not call for heat is the single most common reason a furnace sits silent, and it costs nothing to fix.

Why does my furnace click but not turn on?

A click followed by no flame usually means the igniter is trying and failing to light the burners, most often because the igniter is cracked or worn out. It can also be the gas not reaching the burners. Both are tech repairs, but the igniter is an inexpensive part, so it is worth ruling out before you assume anything bigger.

Can a dirty air filter stop a furnace from turning on?

Yes. A clogged filter starves the airflow, the heat exchanger overheats, and a safety switch shuts the burners off. It often shows up as a furnace that starts then quits, or one that will not stay running. Change the filter, let the unit cool, and try again before calling anyone.

How do I reset my furnace?

On most furnaces there is no dedicated reset button. Flip the furnace power switch off, wait about thirty seconds, and flip it back on to clear the control board and let it restart. If it locks out again right away, it is catching a real fault, not a glitch, and you have moved into service-call territory.

Is it safe to keep restarting a furnace that turns off by itself?

No. A furnace that keeps shutting itself down is doing it on purpose through a safety switch, usually because of overheating from poor airflow or a venting problem. Repeatedly forcing it back on defeats the safety. Clear the obvious airflow issue like a dirty filter once, and if it still trips, leave it off and call a tech.