When your thermostat displays "heating" but your house stays cold, the problem is almost always one of three things: a tripped breaker or blown fuse cutting power to the furnace (takes 30 seconds to check), a dirty flame sensor causing the furnace to ignite then immediately shut off (the #1 furnace service call), or the thermostat fan set to "ON" instead of "AUTO" (blowing unheated air). Start with the 5 free DIY checks below before calling for a service visit.
We've organized all 14 possible causes from easiest/cheapest to fix (check first) to most complex/expensive (check last). About 60% of "no heat" calls can be resolved by homeowners checking items 1–6.
Quick Troubleshooting Checklist
Before diving into details, run through this 2-minute checklist:
- ✅ Is the thermostat set to HEAT mode (not COOL or OFF)?
- ✅ Is the temperature set above current room temperature?
- ✅ Is the fan set to AUTO (not ON)?
- ✅ Is the furnace power switch turned ON?
- ✅ Is the furnace breaker flipped ON at the electrical panel?
- ✅ Is the gas valve open (handle parallel to the pipe)?
If you answered "yes" to all six and there's still no heat, continue to the detailed causes below.
All 14 Causes: Easiest to Hardest
Cause 1: Thermostat Set Incorrectly
This sounds too simple, but it's the cause in about 15–20% of "no heat" calls. Check three settings:
Mode: Make sure the thermostat is set to HEAT, not COOL, AUTO, or OFF. On some smart thermostats, you need to verify the heating schedule is active and the system hasn't been switched to "Away" mode.
Temperature: The set point must be at least 2–3°F above the current room temperature for the furnace to activate. Some thermostats have a built-in temperature differential (called "deadband") that prevents short cycling.
Fan: If the fan is set to ON, the blower runs continuously — even when the burner isn't firing. This blows unheated air from the ducts, making it feel like the system is running but not heating. Switch to AUTO so the blower only runs when the burner is active.
Cause 2: Tripped Circuit Breaker or Blown Fuse
Go to your electrical panel and find the breaker labeled "Furnace" or "HVAC." If it's tripped (in the middle position), flip it fully OFF, then back ON. If it trips again immediately, there's an electrical fault — call a technician.
Note: gas furnaces use electricity for the blower motor, igniter, gas valve, and control board. A tripped breaker kills the entire system even though the fuel is gas.
Cause 3: Furnace Power Switch Off
Every furnace has a dedicated power switch that looks like a standard light switch, usually mounted on or near the furnace, or on a nearby wall. It's commonly mistaken for a light switch and accidentally turned off. Flip it ON.
Cause 4: Clogged Air Filter
A severely clogged filter restricts airflow, causing the furnace to overheat. The high-limit safety switch shuts down the burner to prevent damage. The blower may continue running (blowing cool air) after the burner shuts off. Pull the filter — if it's packed with dirt and you can't see light through it, replace it immediately.
A clogged filter is the #1 cause of preventable furnace shutdowns. It causes the furnace to overheat, short-cycle, and eventually lock out. Check your filter monthly during heating season and replace every 30–90 days. This single habit prevents more service calls than any other maintenance task.
Cause 5: Gas Supply Shut Off
Your furnace's gas valve (a lever on the gas pipe near the furnace) must be in the OPEN position — handle parallel to the pipe. If someone shut it off for maintenance or it was accidentally bumped, no gas reaches the burner. Also check that your gas meter is on and no other gas appliances are affected (if your stove and water heater also have no gas, the issue is at the meter or utility level).
Cause 6: Thermostat Wiring or Dead Batteries
If your thermostat display is blank, flickering, or unresponsive, check the batteries (many programmable thermostats use AA or AAA batteries). Even hardwired thermostats may have backup batteries.
If the display works but the furnace doesn't respond, a loose wire at the thermostat can break the signal. Remove the thermostat from the wall plate and check that the R (red) and W (white) wires are securely attached to their terminals. A loose W wire is a common cause of "thermostat shows heat but furnace doesn't run."
Cause 7: Dirty Flame Sensor
This is the single most common furnace service call. The flame sensor is a thin metal rod that sits in the burner flame. Its job is to verify the burner has lit — if it doesn't detect a flame within 3–7 seconds, it shuts off the gas valve as a safety measure.
A dirty flame sensor (coated with carbon, oxidation, or soot) can't detect the flame even though it's burning. The symptom: the burner ignites, burns for 3–10 seconds, then the flame goes out and the furnace locks out.
DIY fix (experienced homeowners):
- Turn off the furnace and gas.
- Locate the flame sensor — a thin rod with a porcelain base, usually on the opposite side of the burner from the igniter.
- Remove the single screw holding it.
- Gently clean the metal rod with fine-grit sandpaper (emory cloth) or a dollar bill.
- Reinstall, turn on gas and furnace, and test.
This 10-minute fix saves you a $150–$200 service call.
Cause 8: Faulty Hot Surface Igniter
The hot surface igniter (HSI) is a fragile silicon carbide or silicon nitride element that glows red-hot to ignite the gas. After 3–7 years of use, igniters crack or burn out. Signs: you hear the furnace start its ignition sequence, but the burner never lights, and no glow is visible through the sight glass.
Pro repair: $150–$350 including parts and labor. Igniter itself costs $15–$50.
Cause 9: Furnace Lockout
Modern furnaces have control boards that monitor the ignition sequence. If the furnace fails to ignite after 3–4 attempts, the board enters "lockout" mode and stops trying. An LED on the control board flashes an error code (different flash patterns = different errors).
Try this first: Turn the furnace off at the power switch, wait 30 seconds, and turn it back on. This resets the control board and clears the lockout. If the furnace fires up and runs normally, the issue may have been a one-time glitch. If it locks out again, the underlying cause (dirty flame sensor, bad igniter, gas valve issue) still needs to be addressed.
Read your error codes. The furnace's access panel or the manual lists what each flash pattern means. Common codes: 1 flash = system lockout, 2 flashes = pressure switch stuck closed, 3 flashes = pressure switch stuck open, 4 flashes = open high-limit switch. Knowing the code helps you (or your technician) diagnose faster.
Cause 10: Blocked Condensate Drain
On high-efficiency condensing furnaces (90%+ AFUE), a blocked condensate drain triggers the pressure switch, preventing the furnace from firing. The furnace "knows" that if condensate can't drain properly, continued operation could cause water damage or flooding.
See our full guide on furnace leaking water for drain clearing instructions.
Cause 11: Pressure Switch Failure
The pressure switch verifies that the inducer motor is pulling exhaust gases through the heat exchanger. If the switch fails (or the inducer motor is weak), the furnace won't proceed to the ignition stage. You'll hear the inducer motor start (a low humming sound) but the burner never lights.
Pro repair: $150–$400 including parts and labor.
Cause 12: Bad Gas Valve
The gas valve opens to allow gas flow to the burner when the control board signals for heat. A failed gas valve means the igniter glows but no gas reaches the burner. This is relatively uncommon but happens more often in furnaces over 15 years old.
Pro repair: $250–$600 including parts and labor. Gas valve replacement should always be done by a licensed technician.
Cause 13: Blower Motor Failure
If the blower motor fails or runs too slowly, the furnace can overheat because heated air isn't being removed from the heat exchanger. The high-limit switch shuts down the burner. You may hear the burner fire briefly, then shut off, with little or no air coming from the vents.
Pro repair: $300–$800 depending on motor type (single-speed motors cost less; variable-speed ECM motors cost more).
Cause 14: Control Board Failure
The control board is the "brain" of the furnace, sequencing each step of the ignition process. A failed board can cause erratic operation, failure to respond to thermostat signals, or complete shutdown. Often diagnosed by process of elimination after ruling out other components.
Pro repair: $250–$700 including parts and labor.
Real-World Examples
Example 1: The Chens — Thermostat Set to "ON" in Boston, MA The Chens called a technician because "cold air was coming from the vents." The tech arrived, checked the thermostat, and switched the fan from ON to AUTO. Problem solved in 30 seconds. They were charged the $95 service call fee. Lesson: always check thermostat settings first.
Example 2: The Garcias — Dirty Flame Sensor in Denver, CO Their 8-year-old Goodman furnace would start, run for about 5 seconds, then shut off. After three failed attempts, it locked out completely. Mr. Garcia watched a YouTube video, cleaned the flame sensor with emery cloth in 10 minutes, and the furnace ran perfectly. He avoided a $175 service call.
Example 3: The Bakers — Failed Igniter in Indianapolis, IN The Bakers' 6-year-old Carrier furnace showed a blinking error code for "ignition failure." The hot surface igniter had cracked. An HVAC technician replaced it in 30 minutes for $220 total. The igniter was a $35 part — the rest was labor and the service call fee. The furnace ran perfectly after repair.
Example 4: The Nguyens — Gas Valve Stuck Closed in Chicago, IL After trying all the DIY checks, the Nguyens' 18-year-old Rheem furnace still wouldn't heat. The technician found a failed gas valve — the igniter glowed but no gas flowed. Repair cost: $475. Given the furnace's age, the technician recommended replacement instead. The Nguyens opted for a new 96% AFUE furnace, applying the $475 they would have spent on the repair toward the new system.
Key Takeaways
- Check the easy stuff first. Thermostat settings, power switch, breaker, gas valve, and filter account for roughly 60% of "no heat" calls — all free to fix.
- Dirty flame sensor is the #1 professional service call for furnaces. A 10-minute cleaning with fine sandpaper often fixes the problem without a $150–$200 tech visit.
- A clogged air filter can shut down your furnace by triggering the high-limit safety switch. Check it monthly, replace every 30–90 days.
- Read the error codes on your furnace control board — they tell you exactly what's wrong and can save diagnostic time (and money).
- After 3 failed ignition attempts, most furnaces lock out. A simple power-off/power-on reset (30 seconds) clears the lockout, but if the problem recurs, the underlying cause needs repair.
- For causes 8–14 (igniter failure through control board failure), call a licensed HVAC technician. These involve gas lines, high-voltage components, and safety systems that require professional expertise.
- The furnace's age matters. If your furnace is over 15 years old and needs a major component repair ($400+), compare the repair cost against a new furnace installation using the 50% rule.
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