Furnace Not Turning On? 15 Causes and How to Fix Them

Before you start

Trane's owner's guide gives the order for an emergency: if electrical, fuel or mechanical failures occur, close the manual gas valve on the pipe to the furnace first, then turn off the electrical power.⁠[1] The burners light on their own. Bosch warns never to try to light them by hand.⁠[4]

A gas furnace also needs working CO alarms on every level of the home. See where to put CO alarms.

What stops a furnace from starting

A gas furnace runs a safety sequence each time it starts, and any failed check stops it cold. In one Bosch manual there is about a 48-second delay between the thermostat's call and the burners lighting, so give it a minute before you decide nothing is happening.⁠[4] The diagram shows the parts each section below refers to.

1. 01

   Wrong thermostat mode or temperature

   Most commonDIYTime: 2 min

   The thermostat must be on Heat and set above the room temperature.
   Fix it: Wrong thermostat mode or temperature →
2. 02

   Dead thermostat batteries

   Most commonDIYTime: 5 min

   A blank screen often means the batteries ran down.
   Fix it: Dead thermostat batteries →
3. 03

   Power switch off or breaker tripped

   CommonDIYTime: 5 min

   No power at the furnace means no start at all.
   Fix it: Power switch off or breaker tripped →
4. 04

   Blower door not closed

   CommonDIYTime: 2 min

   A safety switch behind the door cuts power when the door is off or ajar.
   Fix it: Blower door not closed →
5. 05

   Loose thermostat wire

   Less commonDIY with careTime: 30 min

   The call for heat never reaches the furnace.
   Fix it: Loose thermostat wire →
6. 06

   Blocked vent pipe or condensate drain

   CommonDIYTime: 15–30 min

   Snow, debris or a plugged drain keeps the burners off.
   Fix it: Blocked vent pipe or condensate drain →
7. 07

   Gas valve to the furnace closed

   Less commonDIYTime: 2 min

   No gas, no flame, even when the furnace tries to start.
   Fix it: Gas valve to the furnace closed →
8. 08

   Status light blinking a code

   CommonDIYTime: 5 min

   The control board is telling you which check failed.
   Fix it: Status light blinking a code →
9. 09

   Dirty air filter

   CommonDIYTime: 10 min

   Low airflow overheats the furnace and trips a safety shutdown.
   Fix it: Dirty air filter →
10. 10

    Worn-out igniter

    CommonPro only

    The small fan runs, but the burners never light.
    What a pro does: Worn-out igniter →
11. 11

    Dirty flame sensor

    CommonPro only

    The burners light, then go out within seconds.
    What a pro does: Dirty flame sensor →
12. 12

    Dirty burners

    Less commonPro only

    Delayed ignition or a yellow flame.
    What a pro does: Dirty burners →
13. 13

    Tripped limit or rollout switch

    Less commonPro only

    An overheating safety switch has shut the burners off.
    What a pro does: Tripped limit or rollout switch →
14. 14

    Blown fuse or failed control board

    Less commonPro only

    Everything outside the furnace checks out, but nothing runs.
    What a pro does: Blown fuse or failed control board →
15. 15

    Electric furnace with no heat

    Less commonPro only

    A failed part inside an electric furnace's control panel.
    What a pro does: Electric furnace with no heat →

Not sure where to start? Answer a few questions and the troubleshooter sends you to the right section. You can also answer a few furnace questions on the full troubleshooter page.

What the furnace does when the thermostat calls for heat tells you where the start-up stalled. The rows follow the order of Trane's problem table and Bosch's start-up sequence.⁠[1]⁠[4]

Check the thermostat first

The thermostat starts every heating cycle. If it doesn't call for heat, a healthy furnace just sits there.

Wrong mode or temperature setting

Carrier's first DIY step is to select heating mode and set the target above the room's current temperature.⁠[5] Resideo's thermostat manual adds one more check: wait 5 minutes for the system to respond.⁠[6]

1. 01

   Step 01 of 03

   Set the system switch to Heat

   • Time: 1 min

   Check that the mode isn't Off or Cool. Some thermostats show "Heat On" while they call for heat.⁠[6]

   You'll know it worked when: The screen shows Heat mode.

2. 02

   Step 02 of 03

   Raise the setpoint above room temperature

   • Time: 1 min

   Set it a few degrees above what the room reads now, so the call for heat is clear.⁠[5]

   You'll know it worked when: The setpoint is a few degrees above the room reading.

3. 03

   Step 03 of 03

   Wait five minutes

   • Time: 5 min

   Many thermostats and furnaces have built-in delays.⁠[6]

   You'll know it worked when: You hear the furnace start, or you move to the next section.

A programmable thermostat changes the temperature on its own schedule, which can make the heat seem to stop at the same time every day. DOE says setting back the thermostat while you sleep or are away can save as much as 10% on heating and cooling, so check the schedule before you blame the furnace.⁠[7]

Still not fixed? Next likely cause: Dead thermostat batteries →

Dead thermostat batteries

Carrier lists dead batteries as one reason a thermostat stops calling the furnace, and Bryant says to replace the batteries in a battery-powered thermostat.⁠[5]⁠[2] On Resideo's RTH2300 series, a "Lo Batt" warning shows for 60 days before the batteries run out, and the manual says to replace them once a year.⁠[6]

1. 01

   Step 01 of 02

   Turn the system switch to Off

   • Time: 1 min

   Resideo says to do this first, or the system could stay on with the batteries removed.⁠[6]

   You'll know it worked when: The thermostat won't keep the furnace running while the batteries are out.

2. 02

   Step 02 of 02

   Fit fresh alkaline batteries

   • Time: 3 min
   • AA alkaline batteries

   Pull the thermostat off its wall plate and match the battery size and direction marked inside. Most settings are stored; you may need to set the clock again.⁠[6]

   You'll know it worked when: The screen lights up and keeps its settings.

If the screen stays blank with new batteries, Resideo's troubleshooting table points to the same places this page checks next: the breaker, the power switch at the furnace, and a furnace door that isn't closed securely.⁠[6]

Still not fixed? Next likely cause: Power switch off or breaker tripped →

A loose thermostat wire

Carrier lists loose wiring as one reason a thermostat stops responding.⁠[5] The thin wires behind the thermostat carry low voltage, but the furnace on the other end does not, so cut the power before you look.

1. 01

   Step 01 of 03

   Turn off the furnace power switch

   • Time: 2 min

   Use the switch on or near the furnace, or its breaker.⁠[2]

   You'll know it worked when: The furnace has no power while you work.

2. 02

   Step 02 of 03

   Look for a wire that has slipped out

   • Time: 10 min
   • LED work flashlight

   Lift the thermostat off its base. Look for a wire that has come loose or a bare end touching its neighbor. Photograph the wiring before you change anything.

   You'll know it worked when: Every wire sits firmly under its terminal.

3. 03

   Step 03 of 03

   Restore power and test

   • Time: 5 min

   Snap the thermostat back on, turn the power on and call for heat.

   You'll know it worked when: The furnace starts when you raise the setpoint.

If a wire is broken inside the wall or you aren't sure which terminal it belongs on, stop and call an HVAC technician. A newly installed thermostat that never worked is a sign of a wiring mismatch, not a furnace fault.

Still not fixed? Next likely cause: Blocked vent pipe or condensate drain →

Power switch off or breaker tripped

A furnace with no power does nothing at all: no fan, no click, no light. Trane's problem table lists a blown fuse or tripped breaker under "no heating."⁠[1] The furnace power switch looks like a light switch and is easy to bump off. American Standard says it is usually in the basement, near the furnace, by the basement stairs or in the ceiling.⁠[8]

1. 01

   Step 01 of 03

   Find the furnace power switch

   • Time: 2 min

   Look on the furnace cabinet and on the walls or ceiling nearby. Bryant says it must be in the On position.⁠[2]

   You'll know it worked when: The switch is in the On position.

2. 02

   Step 02 of 03

   Reset the furnace breaker once

   • Time: 3 min

   In the main panel, a tripped breaker sits in the middle. Carrier says to flip it to Off and then back to On.⁠[5] American Standard suggests checking for loose connections first.⁠[8]

   You'll know it worked when: The breaker stays on and the furnace starts.

3. 03

   Step 03 of 03

   Leave it off if it trips again

   • Time: 1 min

   Bryant says a breaker that trips again should stay off until a professional finds the cause.⁠[2]

   You'll know it worked when: The breaker is off and a pro is booked.

Trane says that unless it is an emergency, never stop the heating system by shutting off the main power. Use the thermostat for everyday control instead.⁠[1]

Still not fixed? Next likely cause: Blower door not closed →

Blower door not closed

Most furnaces have a switch behind the lower front panel. Trane explains that it cuts power to the blower and the gas valve when the door is removed, and the unit must not be altered to run with the door off.⁠[1] American Standard notes that a door left loose after a filter change can stop the furnace from starting.⁠[8]

1. 01

   Step 01 of 02

   Seat the blower door firmly

   • Time: 2 min

   Line up the tabs or hooks and press the door in until it seats. Don't tape the switch or hold it in.

   You'll know it worked when: The door sits flat and its tabs are engaged.

2. 02

   Step 02 of 02

   Call for heat again

   • Time: 5 min

   Trane's fix for an open door is to close it securely to restore power to the blower and gas valve.⁠[1]

   You'll know it worked when: The furnace starts within a few minutes.

The door does more than close a switch. Trane warns that running the furnace with doors removed or ajar can let dangerous fumes escape, so every panel must stay closed.⁠[1]

Still not fixed? Next likely cause: Loose thermostat wire →

Blocked vent pipe or condensate drain

A high-efficiency furnace vents through plastic pipes that often end on an outside wall or the roof. If snow, ice or leaves block them, the furnace won't fire. American Standard tells owners to look for snow, ice, weeds or grass at the venting.⁠[8] NFPA says to keep furnace vents clear of snow and debris.⁠[3]

1. 01

   Step 01 of 02

   Check the vent and intake pipes outside

   • Time: 10 min

   Find where the furnace pipes leave the house. Clear snow, ice and leaves by hand. Trane says the pipe ends must sit at least 12 inches above the normal snow line.⁠[1]

   You'll know it worked when: Both pipe ends are open and clear.

2. 02

   Step 02 of 02

   Look for water at the furnace

   • Time: 2 min
   • LED work flashlight

   Water on the floor, in the drain pan or standing in a clear drain hose points to the next section.

   You'll know it worked when: You know whether the drain is backing up.

Don't cut, move or reroute the vent pipes. Trane warns that changing the venting system can cause carbon monoxide poisoning or death.⁠[1]

When the inducer runs but nothing lights

The inducer is the small fan that pulls exhaust through the furnace before the burners light. A pressure switch has to prove that draft first. In Bosch's chart, an inducer that runs with no further operation points to a blocked flue, a pinched or blocked pressure switch hose, or a weak inducer.⁠[4]

A clear vent outside is the part you can check. The switch, its hose and the inducer are inside the cabinet, and Bosch's corrective steps for them are technician work.⁠[4]

Clogged condensate drain

High-efficiency furnaces make water as they burn gas. Bryant explains that if the drain line clogs, a float switch can keep the furnace off to prevent water damage.⁠[2] Trane's table matches a vent motor that runs with no heat to a plugged condensate drain.⁠[1]

1. 01

   Step 01 of 03

   Turn off the furnace power

   • Time: 1 min

   Use the furnace switch.⁠[1]

   You'll know it worked when: The furnace is off while you work on the drain.

2. 02

   Step 02 of 03

   Clear the drain line and trap

   • Time: 20 min
   • Small wet/dry vacuum

   Trane's owner remedy is to undo the clamps at the condensate trap and drain pan outlet, flush or clean the blockage, and put the clamps back.⁠[1] Keep towels or a wet/dry vac ready for the water.

   You'll know it worked when: Water flows freely out of the drain line.

3. 03

   Step 03 of 03

   Restore power and test

   • Time: 10 min

   Turn the power on and call for heat. Don't bypass the float switch if it keeps the furnace off.⁠[2]

   You'll know it worked when: The furnace runs and the drain stays clear.

Trane says the furnace drain should be checked and cleaned at the start of each heating season.⁠[1] If water keeps backing up, have a technician check the trap and the drain route.

Still not fixed? Next likely cause: Gas valve to the furnace closed →

Gas valve to the furnace closed

Bryant notes that a closed gas valve or a utility interruption will stop the furnace from making heat.⁠[2] Carrier's check is simple: the handle on the valve near the furnace should be parallel to the gas line.⁠[9]

1. 01

   Step 01 of 02

   Look at the gas valve handle

   • Time: 1 min

   Check only the handle position. If you smell gas at any point, leave and call from outside.⁠[9]

   You'll know it worked when: The handle runs in line with the gas pipe.

2. 02

   Step 02 of 02

   Check another gas appliance

   • Time: 2 min

   If the stove or water heater also won't light, Carrier says the gas supply itself may be disrupted. Call your gas company.⁠[5]

   You'll know it worked when: You know whether the whole house has gas.

Bryant says never to repair a gas valve yourself.⁠[2] If the handle was closed and you don't know why, ask a technician to check the furnace before you open it.

Still not fixed? Next likely cause: Status light blinking a code →

Status light blinking a code

Many furnaces have a small light on the control board, visible through a window in the blower door. Bosch's control flashes a set pattern that matches a fault, and its chart separates pressure switch, limit switch and ignition lockouts.⁠[4] Codes differ by brand and model, so Bryant says to write down the pattern and check the owner's manual.⁠[2]

1. 01

   Step 01 of 03

   Count the flashes

   • Time: 3 min

   Watch through the window without opening the door. Count the flashes between pauses, twice.

   You'll know it worked when: You have the number of flashes and the color of the light.

2. 02

   Step 02 of 03

   Match the pattern to your manual

   • Time: 5 min

   Use the code chart on the inside of the blower door or in the owner's manual. Carrier notes that some error codes call for professional service.⁠[9]

   You'll know it worked when: You know which safety check stopped the furnace.

3. 03

   Step 03 of 03

   Reset a lockout once

   • Time: 5 min

   Bosch's control clears a lockout after a power interruption of 1 to 20 seconds, or on its own after one hour. If the cause is still there, it locks out again.⁠[4]

   You'll know it worked when: The furnace tries a new start.

A lockout protects you. Bosch's control goes into lockout after three failed ignition tries, so a furnace that locks out again and again needs a technician, not more resets.⁠[4]

Still not fixed? Next likely cause: Dirty air filter →

Dirty air filter

A filter packed with dust starves the furnace of air. Carrier says this stresses the heat exchanger and can cause overheating and an automatic shutdown.⁠[5] The furnace may start, run a short time, then quit. Bosch calls poor filter care the most common cause of weak heating or cooling.⁠[4]

1. 01

   Step 01 of 03

   Turn off the furnace power

   • Time: 1 min

   Trane says to disconnect power before you take off the blower door.⁠[1]

   You'll know it worked when: The blower can't start while the filter is out.

2. 02

   Step 02 of 03

   Check the filter against a light

   • Time: 3 min

   Slide it out of its slot or rack. If little light passes through, it is due.

   You'll know it worked when: You know whether the filter is clogged.

3. 03

   Step 03 of 03

   Fit a filter of the same size and type

   • Time: 5 min
   • Furnace air filter

   Bosch says to use the same type and size, with the airflow arrow toward the blower.⁠[4] Close the door and restore power.

   You'll know it worked when: The new filter sits snug, with its arrow pointing toward the blower.

Carrier recommends checking the filter monthly, and replacing many 1-inch filters every 30 to 90 days.⁠[9] Trane says never to run the furnace with the filter removed.⁠[1]

Still not fixed? Next likely cause: Worn-out igniter →

Worn-out igniter

The igniter lights the burners at every call for heat. Carrier notes that ignitors wear out from frequent cycling.⁠[9] Bryant explains that if a modern igniter cracks or fails, the gas will not ignite.⁠[2] A typical sign: the small inducer fan runs, but the burners never light.

Leave the igniter to a technician. Trane warns not to touch it because it is extremely hot, and calls it very fragile.⁠[1] Bosch lists ignitor service for qualified service technicians only.⁠[4] For what you can safely watch for, see furnace ignitor not glowing.

Still not fixed? Next likely cause: Dirty flame sensor →

Dirty flame sensor

The flame sensor proves the burners are lit. Bryant says that when it is coated in carbon or dust, it shuts the gas valve as a precaution.⁠[2] The burners light, then go out within a few seconds, and the furnace tries again.

Bosch says the coating can be nearly invisible and that a qualified service technician should clean the sensor.⁠[4] Repeated start-and-stop cycles are a reason to book service.⁠[2]

Still not fixed? Next likely cause: Dirty burners →

Dirty burners

Trane says dirt on the burners can cause a yellow flame or delayed ignition, and either one means a service call.⁠[1] Carrier adds that a bang or boom at start-up suggests a clogged burner delaying ignition.⁠[9]

Watch the flame through the window only. Bosch describes a healthy flame as stable, quiet, soft and blue, and says to call a qualified service agent at once if it looks abnormal.⁠[4]

Still not fixed? Next likely cause: Tripped limit or rollout switch →

Tripped limit or rollout switch

Limit switches shut the burners off when the furnace overheats. Bryant explains that a clogged filter or blocked vents can restrict airflow and trip them.⁠[2] Bosch adds manual-reset rollout switches that open if flames are not drawn properly into the heat exchanger.⁠[4]

Bryant says a limit switch should not be bypassed and repeated shutdowns need professional diagnosis.⁠[2] Bosch says to fix the cause before any switch is reset.⁠[4] What you can do is keep the filter clean and the registers open, which protects the furnace from overheating.

Still not fixed? Next likely cause: Blown fuse or failed control board →

Blown fuse or failed control board

If the thermostat, power and door all check out and the furnace still does nothing, the fault is inside. Bryant notes that older furnaces often use internal fuses, and that control board problems need professional diagnosis.⁠[2] Bosch's chart traces a furnace with no status light to missing power, an open door switch, a blown fuse or a control fault.⁠[4]

Board and fuse work means opening the control compartment, which is technician work. Write down what you already checked so the visit starts there.

Still not fixed? Next likely cause: Electric furnace with no heat →

Electric furnace with no heat

An electric furnace has no burners or gas valve. It switches heating elements on from a control panel. Nordyne's owner manual says to check that all power supply switches are on and the furnace door is closed and latched.⁠[10] Some electric furnaces have their own breakers on the cabinet, so check those too.⁠[10]

The same manual says there are no user-serviceable parts in the control panel, and not to open it.⁠[10] If the breakers are on and the door is latched, call an HVAC technician.

Still not fixed? See when to call a pro →

Parts and tools for this fix

Most checks need no tools at all. Batteries and a flashlight cover the thermostat and the cabinet checks, and a wet/dry vac helps with a drain that has backed up.⁠[1]

A clean filter prevents a lot of no-heat calls. Bryant says a clogged filter can overheat the heat exchanger and trigger a safety shutdown.⁠[2] Before cold weather, run through the pre-winter furnace checks. If the furnace is old and the repair is big, use the calculator to repair or replace an old furnace.

When to call a pro

Trane's guide is blunt: other than the simple maintenance it describes, owners should not make adjustments to a furnace. Your dealer or a qualified service agency handles the rest.⁠[1]

Give the technician the flash pattern, what you already checked, and the model and serial numbers from the rating plate. Bosch asks for both numbers on parts orders.⁠[4]

Frequently asked questions