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Fix a furnace that will not turn on

We'll confirm the thermostat is calling, rule out power and filter blockage, then isolate the cause—limit switch, thermostat, or heating elements for electric; pilot or igniter for gas—or tell you when to call a pro.

Troubleshooting · Home appliances
15–45 min
What you'll need
  • Multimeter (for continuity tests on electric furnaces)
  • Screwdriver set (Phillips and flathead)
  • New air filter (if current one is clogged)
  • Replacement igniter (if tests show a fault on gas furnace)
Step 1 of 10

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Steps

Goal: Confirm the thermostat is calling, rule out power and filter, then isolate the fault.

  • Set the thermostat to heat and raise the temperature above the current room temperature. Check the display for heat mode and that the setpoint is higher than the room reading.
  • Good: The thermostat shows heat mode and setpoint above room temp. Proceed to Check power and filter.
  • Bad: The thermostat does not respond or shows no display—replace batteries or check wiring. See When to get help.

Check power and filter

Goal: Rule out power loss and filter blockage before opening the furnace.

  • Verify the circuit breaker for the furnace has not tripped. Check the furnace power switch—many furnaces have a switch on the unit or on a nearby wall that looks like a light switch. Confirm both are on.
  • Locate the air filter (usually in the return duct or at the furnace). Remove it and hold it to the light—if you cannot see light through it, replace it. Install a new filter with the arrow pointing toward the furnace.
  • Good: Power is on and filter is clean or replaced. Proceed to Electric path or Gas path based on your furnace type.
  • Bad: Breaker keeps tripping or filter was severely clogged—fix those first, or call a pro if the breaker trips immediately.

Electric path

Goal: Test and reset the limit switch, thermostat, and heating elements on an electric furnace.

  • Turn off power at the breaker. Check the limit switch—a safety that opens when the furnace overheats. Replace the filter first if you have not already. Look for a reset button per your model. Press it if found. Test for continuity with a multimeter.
  • If the limit switch is closed or reset, test the thermostat for continuity when set to call for heat. Test each heating element. Replace any that show no continuity.
  • Good: You found and replaced the faulty part. Restore power and test—the furnace should respond when the thermostat calls for heat.
  • Bad: The limit switch keeps tripping after a clean filter, or you are not comfortable with electrical work—call an HVAC technician.

Gas path

Goal: Check gas supply and pilot or igniter on a gas furnace. Gas valve work requires a pro.

  • Confirm the gas supply valve is on (handle parallel to the pipe). Check the furnace gas valve per the label. If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside.
  • Older gas furnaces: Remove the access panel and look for the pilot light. If the pilot is out, call an HVAC technician—do not try to restore the pilot flame yourself.
  • Modern gas furnaces: When the thermostat calls for heat, the hot surface igniter should glow. If it does not glow, replace it. If it glows but no flame appears, the gas valve has failed—call an HVAC technician.
  • Good: The pilot is lit or the igniter glows and the burner lights. The furnace heats.
  • Bad: The igniter glows but no flame—gas valve issue. Call an HVAC technician. Do not work on gas valves yourself.

When to get help

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside. Do not call a technician for an active gas leak—that is an emergency.

Call an HVAC technician if:

  • The pilot will not stay lit (thermocouple or gas valve).
  • The igniter glows but no flame appears (gas valve).
  • You have checked power and filter and the furnace still does not respond.
  • The limit switch keeps tripping after a clean filter.
  • You are not comfortable working with electricity or gas.

Verification

  • The thermostat calls for heat and the furnace responds—blower runs and warm air comes from the vents.
  • No error codes or unusual noises.
  • The limit switch, thermostat, or igniter (if replaced) works and the furnace runs consistently when heat is needed.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm thermostat Verify the thermostat is set to heat and calling; rule out user error.
  2. Power and filter Check circuit breaker, furnace switch, and air filter; both can prevent the furnace from running.
  3. Electric — limit switch and thermostat Check and reset limit switch; test thermostat and heating elements if needed.
  4. Gas — pilot or igniter Check gas supply; inspect pilot or igniter; replace igniter if cracked or failed.
  5. Call a pro Gas smell—evacuate, call 911 or gas utility from outside. Gas valve work or repeated failures—call an HVAC technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Furnace type (electric or gas)
  • Whether the thermostat is calling for heat
  • Whether the filter was clogged
  • Limit switch / thermostat / igniter test results
  • Steps already tried

Is the thermostat set to heat and calling for heat?

Set the thermostat to heat and raise the temperature above room temp. Check for low-battery indicator on battery-powered units.

Set the thermostat to heat and raise the setpoint above the current room temperature. Check the display for heat mode and that the setpoint is higher than the room reading. Good: thermostat shows heat mode and setpoint above room temp. Bad: thermostat does not respond or shows no display—replace batteries or check wiring.

Yes No

You can change your answer later.

Fix thermostat first

Replace thermostat batteries if battery-powered. Check that the thermostat is set to heat (not cool or off) and the setpoint is above room temperature. If the thermostat still does not respond, the wiring or thermostat may be faulty—call an HVAC technician.

Done

Is power on and the filter clean?

Circuit breaker, furnace switch, and clogged filter can prevent the furnace from running.

Verify the circuit breaker is on and the furnace power switch (on the unit or nearby wall) is on. Check the filter—if you cannot see light through it, replace it. A clogged filter trips the limit switch. Good: power on and filter clean or replaced. Bad: breaker keeps tripping or filter severely clogged—fix those first.

Yes No

You can change your answer later.

Is the furnace electric or gas?

Electric furnaces have a power cord or junction box. Gas furnaces have a gas line and burner assembly.

Check the furnace. Electric: thick power cord or junction box, heating elements. Gas: gas line, vent pipe, burner. Good: you know the type. Bad: unsure—call an HVAC technician.

Electric Gas

You can change your answer later.

Is the limit switch closed or reset?

The limit switch opens when the furnace overheats. A clogged filter is a common cause. Some have a manual reset.

Check the limit switch. Replace the filter first if you have not already. Look for a reset button per your model. Press it if found. Test for continuity with a multimeter. Closed or reset: proceed to thermostat and elements. Keeps tripping: call an HVAC technician.

Yes No (keeps tripping)

You can change your answer later.

Do the thermostat and heating elements have continuity?

Test the thermostat when calling for heat. Test each heating element.

Turn off power. Test the thermostat for continuity when set to call for heat. Test each heating element. Replace any that show no continuity. Good: replaced or all test good—reassemble and test. Bad: not comfortable with electrical work—call an HVAC technician.
Question

Do the thermostat and elements test good?

Done

Is the gas supply on?

The gas valve must be on. Handle parallel to pipe = on.

Check the gas supply valve. Handle parallel to the pipe = on. Check the furnace gas valve per the label. Good: gas on. Bad: if you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside.

Yes No (or smell gas)

You can change your answer later.

Is the pilot lit or does the igniter glow?

Older furnaces have a standing pilot. Modern ones use a hot surface igniter.

Older: remove access panel, look for pilot light. If out, call an HVAC technician—do not try to restore it yourself. Modern: when thermostat calls for heat, the hot surface igniter should glow. If it does not glow, replace it. If it glows but no flame, gas valve has failed—call a technician.
Question

Is the pilot lit or does the igniter glow and flame light?

Continue

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Replace igniter and test

Replace the igniter with a matching part. Restore power. When the thermostat calls for heat, the igniter should glow and the burner should light. If the igniter glows but no flame appears, the gas valve has failed—call an HVAC technician.

Done

Call a technician

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside—do not call a technician for an active gas leak. Call an HVAC technician if: the pilot will not stay lit; the igniter glows but no flame (gas valve); you have checked power and filter and the furnace still does not respond; or you are not comfortable working with electricity or gas. Never work on gas valves or gas lines yourself.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a furnace not turn on at all?
Electric furnaces: a tripped limit switch, faulty thermostat, or failed heating elements. Gas furnaces: pilot out, failed igniter, or gas valve. A clogged air filter can trip the limit switch and prevent the furnace from running. Check thermostat, power, and filter first.
Can I fix a furnace that will not turn on myself?
Yes, you can check the thermostat, circuit breaker, furnace switch, and air filter. You can replace a dirty filter and reset a tripped limit switch. Gas furnaces: you can inspect the pilot or igniter, but gas valve work requires a professional.
When should I call an HVAC technician for a furnace that will not turn on?
If you smell gas, evacuate immediately and call 911 or your gas utility from outside—do not call a technician for an active leak. Call an HVAC technician if the pilot will not stay lit, the igniter glows but no flame appears (gas valve), you have checked power and filter and the furnace still does not respond, or you are not comfortable working with electricity or gas.

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