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Diagnostic guide

Furnace Not Heating

Diagnose a gas furnace that will not fire, short cycles, or blows cold air, starting with what you can safely check.

System
Heating
Causes covered
9
You can check
4 without a technician

Stop and read this first

  • If a carbon monoxide alarm sounds, leave the building and call 911 from outside. Do not re-enter to investigate.
  • If you smell gas, do not operate switches or phones indoors. Leave and call the gas utility's emergency line from outside.
  • Do not attempt to reset a furnace repeatedly after a lockout. Lockouts exist to prevent unburned gas accumulating.
  • Never bypass a safety switch to keep a furnace running, no matter what you read online.

When to call a licensed technician

These situations are beyond safe DIY troubleshooting. A service call is cheaper than a misdiagnosis that damages equipment or creates a safety hazard.

  • You smell gas or a carbon monoxide alarm sounds — leave the building and call from outside.
  • The burner lights then shuts off within seconds, repeating until lockout.
  • You see soot around the burners or flames that move when the blower starts.
  • The furnace is locked out and will not restart after one reset attempt.
  • Any repair requires opening the gas valve, heat exchanger, or electrical compartment.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Thermostat settings or dead batteries

    CommonYou can check
    How to check
    Mode set to Heat, setpoint above room temperature, display lit and responsive.
    The fix
    Replace batteries and correct the mode. Raise the setpoint five degrees and listen for the system to call.

  2. Clogged air filter causing limit shutdown

    CommonYou can check
    How to check
    The furnace fires, runs briefly, then shuts down and repeats. The filter is visibly loaded.
    The fix
    Replace the filter. A restricted filter causes the high-limit switch to trip on overheat, which is the most common cause of short cycling.

  3. Furnace power switch or door safety switch open

    CommonYou can check
    How to check
    Check the switch on or near the furnace, and confirm the blower access door is seated fully.
    The fix
    Turn the switch on and reseat the door. The door interlock will not allow the furnace to run if the panel is not fully closed.

  4. Blocked condensate drain on a high-efficiency furnace

    CommonYou can check
    How to check
    Water near the furnace, or a unit that locks out with the inducer running but no ignition.
    The fix
    Clear the condensate trap and drain line. A blocked trap trips the pressure switch and prevents ignition.

  5. Failed igniter

    CommonNeeds a pro
    How to check
    The inducer runs, the gas valve does not open, and you never see a glow inside the burner compartment.
    The fix
    A technician replaces the hot surface igniter. This is a routine wear item, typically failing every five to ten years.

  6. Dirty flame sensor

    CommonNeeds a pro
    How to check
    The burner lights and then shuts off after three to seven seconds, repeating several times before lockout.
    The fix
    A technician cleans the flame rod. This is one of the most common and least expensive furnace service calls.

  7. Pressure switch or blocked flue

    PossibleNeeds a pro
    How to check
    The inducer motor runs but ignition never begins. Check the exterior flue termination for a bird nest, snow or ice.
    The fix
    Clear an obstructed termination if it is safely reachable. Otherwise a technician tests the switch and verifies draft.

  8. Gas supply interrupted

    PossibleNeeds a pro
    How to check
    Confirm other gas appliances work. Check that the furnace gas valve is open.
    The fix
    If no gas appliance works, contact the utility. If only the furnace is affected, a technician checks the valve and gas pressure.

  9. Cracked heat exchanger

    UncommonNeeds a pro
    How to check
    A carbon monoxide alarm, flames that visibly move when the blower starts, or soot around the burners.
    The fix
    Shut the furnace off at the switch and ventilate. This is a carbon monoxide hazard and almost always means the furnace is replaced, not repaired.

Direct answer

A furnace that will not heat usually traces to thermostat settings, a tripped breaker or switch, a dirty filter tripping the limit, or ignition failure on gas units (dirty flame sensor, bad igniter, closed gas valve). Verify heat mode, fan on auto, and that the filter is clean before calling for service. If you smell gas, hear repeated clicking with no flame, or see a red blinking error code, stop troubleshooting and schedule a licensed HVAC technician.

Severity and urgency

Fix now when heat is required

No heat during freezing weather is a health and pipe-freeze risk within hours. Gas ignition faults and CO risks require professional diagnosis — do not bypass safety switches.

Cost of ignoring it

Frozen pipes from no heat can cause $5,000–$20,000 water damage. A weak igniter left failing can fill a heat exchanger with unburned gas on some designs. Running with a cracked heat exchanger risks carbon monoxide exposure.

How a pro will diagnose it

Techs read fault codes, measure gas pressure, inspect the inducer, test the flame sensor and igniter, and verify limit switches reset properly. They should combustion-test gas furnaces before leaving. Diagnostic fees run $89–$199 and often credit toward repair. You should get pass/fail on heat exchanger inspection if rollout switches tripped.

Prevention and maintenance

  • Replace filters monthly in heating season
  • Keep return paths clear; never store items against the furnace
  • Annual fall tune-up including flame sensor cleaning and combustion check
  • Test CO alarms and replace batteries before heating season
  • Know where the gas shutoff and furnace switch are — label them

For trade owners

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Common questions

Why is my furnace not heating?
Start with the thermostat (mode, batteries, setpoint), then the air filter — a clogged filter is the most common cause of short cycling and no heat. Next check the furnace power switch, blower door, and condensate drain on high-efficiency units. If the inducer runs but the burner never lights, the igniter or flame sensor likely needs service.
Furnace running but blowing cold air — what should I check?
Confirm the thermostat is in Heat mode with the setpoint above room temperature. A dirty filter can cause the furnace to overheat and shut down the burner while the blower keeps running. If the burner lights briefly then shuts off, the flame sensor or a blocked flue is the likely cause — both need a technician.
How much does it cost to fix a furnace that will not heat?
Common fixes run $150 to $400 for a flame sensor cleaning, $200 to $500 for an igniter replacement, and $300 to $800 for a pressure switch or inducer motor. If the heat exchanger is cracked, replacement runs $3,500 to $6,500 — repair is rarely recommended past 15 years.
When should I replace a furnace instead of repairing it?
Replace when the unit is over 15 years old, faces a repair over $1,500, needs R-22 or obsolete parts, or has a cracked heat exchanger. A furnace past 20 years with any major component failure usually costs more to keep alive than to replace.
Why does my furnace keep running?
If the blower runs without heat, the thermostat fan may be set to On instead of Auto — that is a free fix. If heat never comes on, start with the filter and ignition causes in this guide. If only the fan runs continuously with no heat call, use the furnace blower troubleshooting guide for relay, wiring, and limit switch causes.
Why is my furnace not turning on?
Check thermostat heat mode, power switch at the furnace, breaker, and filter. Many furnaces lock out after failed ignition attempts — power cycle once after fixing obvious issues, then call a pro.
Furnace clicks but no heat?
Common on gas units: worn hot-surface igniter, dirty flame sensor, or blocked flue tripping the pressure switch. Inducer motor failure also prevents ignition sequence.
Could a dirty filter stop heat?
Yes — restricted airflow trips the high-limit and the furnace shuts down to prevent overheating. Replace filter and wait 30 minutes for limit reset.
Is no heat an emergency?
In sub-freezing weather with vulnerable occupants, yes. Otherwise schedule same-day or next-day. Gas smell is always emergency — leave and call the utility.
What does furnace repair cost for no heat?
Flame sensor cleaning or igniter: $100–$250. Inducer motor: $400–$700. Control board: $400–$900. Heat exchanger failure: replacement $3,500–$6,500.
Why heat works sometimes but not others?
Intermittent issues point to failing igniters, loose wiring, overheating from marginal airflow, or a sticking gas valve — all need pro diagnosis before they fail fully on a cold night.
Should I reset the furnace breaker?
Once, if you found no obvious gas smell or scorch marks. Repeated trips mean leave it off — electrical or motor faults need a technician.
Can a smart thermostat prevent heating?
Yes — wrong wiring (no C-wire), dead battery backup, or schedule hold in away/eco. Verify the thermostat shows an active heat call.
Electric furnace not heating?
Check sequencers, heating elements, and breakers — often two-pole 60A. Elements fail open; sequencers stick. Requires electrical testing.
When is replacement smarter than repair?
Heat exchanger crack, repeated major component failures on a unit over 15–20 years, or repair quote above 50% of replacement — run repair-vs-replace math.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

Furnace Replacement cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Related symptoms

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