Most furnace repair comes down to one part failing at one step of the startup sequence: a dirty flame sensor, a cracked hot surface ignitor, a pressure switch that never closes, or a limit switch tripping because airflow is restricted. These are usually single-part repairs finished in one visit. The big exception is a cracked heat exchanger, which is a carbon monoxide risk and, on an older furnace, usually the end of it.
This guide covers modern gas furnaces with electronic ignition. If you haven't narrowed the problem to the furnace yet, start with heating repair by system type.
How a Gas Furnace Starts, and What Fails at Each Step
Every modern gas furnace runs the same sequence of operation. Knowing where it stops tells you, and the technician, which part to look at.
- Call for heat. The thermostat sends 24 volts to the control board. Fails at: thermostat settings, a broken wire, a tripped breaker, or a blown board fuse.
- Safety check. The board confirms the limit and rollout switches are closed. Fails at: an open limit or rollout switch.
- Inducer motor starts and pulls exhaust through the heat exchanger and up the vent. Fails at: a worn or seized inducer, often noisy first.
- Pressure switch closes, proving the vent drafts before gas flows. Fails at: a blocked vent, a cracked or water-filled hose, a clogged condensate trap, or a weak inducer.
- Ignitor glows orange for several seconds. Fails at: a cracked hot surface ignitor.
- Gas valve opens. Fails at: a bad valve, a closed manual shutoff, or no gas supply.
- Flame sensor proves flame within seconds. Fails at: a sooty sensor or poor ground. The burners light and drop out.
- Blower starts after a short delay. Fails at: the blower motor, its capacitor, or the board's relay. With no blower, the limit switch trips and kills the burners.
Most boards retry ignition a few times, then lock out. Many wait about an hour before trying again, which is why a furnace can seem to fix itself and then fail again.
Read the Control Board Flash Code First
Most furnaces made in the last few decades have an LED on the control board that blinks a diagnostic code, visible through a small window in the blower door. Count the flashes before you touch anything, because cutting power often clears the stored code. Match the count to the chart printed on the inside of the door panel or in the installation manual.
Codes are brand-specific: three flashes on a Goodman furnace don't mean the same thing as three flashes on a Lennox, so use the chart for your model. Typical codes cover the pressure switch, an open limit or rollout switch, ignition lockout and reversed polarity. Write the code down for whoever does the repair.
Furnace Symptoms and Likely Causes
| What you notice | Most likely cause | Who handles it |
|---|---|---|
| Nothing happens, no inducer, no light on the board | Thermostat, breaker, service switch, furnace door not seated, blown board fuse | Power and thermostat checks are yours. A blown fuse means a short and needs a tech |
| Inducer runs, then stops, no ignition | Pressure switch not closing: blocked vent, clogged condensate trap, cracked hose, weak inducer | Check vent pipes outside. The rest is a tech job |
| Ignitor never glows | Cracked ignitor, or the board isn't sending power to it | Tech |
| Burners light, then go out in 3 to 10 seconds | Dirty or failing flame sensor, weak ground | Tech, or a careful homeowner for cleaning |
| Burners run a few minutes, then shut off and the blower keeps going | High-limit switch tripping on overheating, usually a clogged filter or closed vents | Filter is yours. A repeat trip with a clean filter needs a tech |
| Blower runs constantly, air is cool | Thermostat fan set to On, or the burner has locked out | Check the fan setting, then read the flash code |
| Loud bang when the burners light | Delayed ignition from dirty burners or weak gas pressure | Tech, soon. This stresses the heat exchanger |
| Squealing, grinding or humming from the blower | Worn bearings or a failed run capacitor | Tech. See furnace blower motor symptoms and replacement |
| CO alarm sounds, soot, or a wavering flame | Possible cracked heat exchanger or venting problem | Leave the house. Call 911 or the gas utility from outside |
What You Can Safely Check Before Calling Anyone
These cost nothing and solve a lot of no-heat calls:
- Thermostat. Set to Heat, a few degrees above room temperature, fresh batteries.
- Power. Check the breaker and the service switch near the furnace, which looks like a light switch.
- Furnace door. The blower door presses a safety switch. If it isn't fully seated, nothing runs.
- Filter. A clogged filter causes overheating and limit trips. Replace it if you can't see light through it. See how to change a furnace filter.
- Gas supply. The gas valve handle at the furnace should be parallel to the pipe.
- Vent pipes. On a high-efficiency furnace with white PVC pipes, check the outside terminations for snow, ice, leaves or nests.
- Condensate drain. On high-efficiency furnaces, a plugged trap or drain line can keep the pressure switch open.
- Registers and returns. Open supply registers and clear furniture off return grilles.
After the checks, reset power once at the service switch and watch one full cycle. If it fails the same way, stop and call someone.
What Not to Touch
- Never bypass or jumper a limit, rollout or pressure switch. They exist to stop overheating, flame rollout and exhaust spilling into the house. A furnace running with a jumpered safety can crack its heat exchanger or start a fire.
- Don't keep resetting a rollout switch. A tripped rollout means flame came out of the burner area. Reset it once at most, and if it trips again, shut the furnace off.
- If you smell gas, don't touch switches or the thermostat. Leave the house and call the gas utility from outside.
- Leave the gas valve, burners, wiring and capacitors alone. Run capacitors hold a charge after the power is off.
Cleaning a flame sensor with a fine abrasive pad is a common homeowner task with the power and gas off, but if you aren't comfortable removing a part from the burner assembly, leave it to the technician.
What Common Furnace Repairs Involve
Flame sensor. Clean the rod, check the ground, measure the microamp flame signal. Replace only if it's cracked or still reads weak. The furnace flame sensor guide covers cleaning and why some sensors keep fouling.
Hot surface ignitor. Confirm failure with a resistance test, then fit a matched ignitor. They're brittle, so careful handling matters. See how a furnace ignitor fails for telling it apart from a pressure switch fault.
Pressure switch and inducer. Check the vent, hose and condensate trap first, then measure the draft the inducer makes. Inducers are often replaced as an assembly.
Limit and rollout switches. A limit switch is rarely the root problem. The tech should find the airflow restriction behind it. A rollout trip calls for inspecting the burners, flue and heat exchanger. The furnace limit switch guide explains each switch type.
Blower motor. Measure the capacitor and amp draw before condemning the motor. PSC and ECM motors differ widely in price.
Control board. Boards fail from age, heat and shorts. A good tech finds out why a board or its fuse failed before installing a new one.
Cracked Heat Exchanger and Carbon Monoxide
The heat exchanger keeps combustion gases out of the air you breathe. Years of expansion and contraction, chronic overheating from poor airflow, and corrosion can crack it, letting carbon monoxide into the house air.
A technician diagnoses it with a camera inspection, a combustion analysis, and by watching for a flame that wavers when the blower starts. If a tech finds a crack, the furnace may be red-tagged and must stay off. Ask to see photos or video of the crack, and on a borderline call, get a second opinion before spending replacement money. Our cracked heat exchanger guide covers the signs, the warranty question and what to ask.
Keep working CO alarms on every level of the home and near sleeping areas. If one sounds, get everyone outside and call 911 or the gas utility.
What Drives Furnace Repair Cost
The bill depends more on which part failed than anything else:
- The part. A flame sensor cleaning sits at the low end. Ignitors and pressure switches fall in the lower middle. Inducer motors, blower motors and control boards land in the middle to high range. A heat exchanger is often the single most expensive furnace repair and usually not worth doing outside the warranty.
- Furnace type. Two-stage and modulating furnaces with variable-speed blowers use more expensive, often OEM-only parts.
- Warranty status. Many manufacturers cover parts for years when the furnace was registered. Labor usually isn't covered.
- Access and timing. Attic and crawlspace installs take longer. Nights, weekends and the first cold snap of the season cost more.
- Diagnostic fee. Most companies charge one and many credit it toward the repair.
For the replacement side of the math, use the furnace replacement cost guide.
Repair or Replace Your Furnace?
Gas furnaces commonly last 15 to 30 years, with most replaced somewhere in the middle of that range. The 50 percent rule is a reasonable starting point: if the repair costs more than half of a new furnace and the unit is past the middle of its life, replacement usually wins.
A few situations override the math:
- A confirmed cracked heat exchanger on an out-of-warranty furnace almost always means replacement.
- Repeat failures, such as a second major part in the same season.
- Discontinued parts that would leave you waiting in midwinter.
- Low efficiency. An older 80 percent AFUE furnace costs more to run than a 95 percent unit.
Regular annual furnace maintenance catches most small failures before they turn into big ones.
How to Vet a Furnace Repair Contractor
Questions to ask:
- What flash code did the board show, and what did you measure to confirm the failed part?
- Is the replacement part OEM or universal, and is the furnace still under a parts warranty?
- Did you find the cause, such as the airflow problem behind a limit trip?
- If you're calling the heat exchanger cracked, can you show me the photos or the combustion readings?
- What is the written warranty on parts and labor, and does the diagnostic fee apply to the repair?
Red flags:
- Condemning the furnace without showing evidence of a crack.
- Quoting a new furnace before measuring anything.
- Offering to bypass a safety switch "to get you through the night."
- Pressure to sign for a replacement the same day.
Frequently Asked Questions
What is the most common furnace repair?
A dirty flame sensor, which makes the burners light and drop out within seconds. Cracked hot surface ignitors, pressure switch faults and limit trips from a clogged filter follow close behind. All are relatively inexpensive.
Why does my furnace turn on and then shut off after a few seconds?
Burners that go out within about 3 to 10 seconds mean the board isn't seeing a flame signal, usually a dirty flame sensor. A shutdown after a few minutes points at the high-limit switch tripping on poor airflow, often a clogged filter.
Is it worth repairing a 20-year-old furnace?
A cheap repair like a flame sensor or ignitor still is. A control board, inducer or blower motor deserves a replacement quote alongside it, and a cracked heat exchanger at that age almost always means replacement.
How do I know if my furnace heat exchanger is cracked?
You usually can't confirm it yourself. Warning signs include a CO alarm, soot around the burners, a flame that wavers when the blower starts, and repeated rollout trips. A technician confirms it with a camera or combustion analysis. Ask to see the evidence.
Can I fix my furnace myself?
You can check the thermostat, breaker, filter, furnace door, vent pipes and condensate drain, and read the flash code. Gas valves, burners, wiring and safety switches belong to a technician. Never bypass a limit, rollout or pressure switch.
If the furnace won't start at all and you're still sorting out why, the step-by-step checks in why a furnace stops working go further on each no-heat scenario.