Furnace repair - safety first, parts second
Most furnace faults are cheap parts. A few are carbon monoxide. Knowing which one you are looking at is the whole point of this page.
What is furnace repair?
Start with safety. If your carbon monoxide alarm has sounded, if you smell gas, or if anyone in the house has unexplained headaches or nausea that improve when they leave - get everyone outside and call the utility or the fire service before you call a repair company. Everything else is a normal repair. The usual culprits are a dirty flame sensor, a failed hot surface ignitor, a blocked condensate line, a failing inducer motor or a tripped limit switch - all ordinary, none of them expensive.
Signs you need furnace repair
- The blower runs but only cold air comes out - often a failed ignitor or a flame sensor the burner cannot prove
- The furnace lights, runs for a minute or two, then shuts down and tries again - a classic dirty flame sensor pattern
- It clicks repeatedly and never fires at all
- The burner flame is yellow, flickering or lifting off, rather than a steady blue
- Soot, rust streaks or scorch marks anywhere on or around the cabinet or the flue pipe
- A high-efficiency furnace that keeps shutting off - very often a blocked condensate drain triggering the float switch
- A rhythmic squeal or grinding from the blower compartment
- A carbon monoxide alarm that has sounded, even once, even if it stopped
Seeing something not on this list? Browse hvac problems by symptom →
How furnace repair actually works
Safety check before diagnosis
A competent technician looks at the flue connection, the heat exchanger, the burner flame and the draft before touching a part. Combustion equipment gets a safety assessment first, because the cheap fix is irrelevant if the appliance should not be running at all.
Watch forA technician who goes straight to swapping parts without inspecting the heat exchanger or the venting. On a fuel-burning appliance that is the wrong order of operations.
Diagnosis of the fault itself
Most modern furnaces store a fault code and flash it on a control board LED. From there the technician confirms with instruments: microamp reading on the flame sensor, resistance on the ignitor, pressure switch operation, limit switch continuity, inducer amp draw.
Watch forA diagnosis given with no reference to the board's fault code. The furnace is telling you what it thinks is wrong; a technician who does not read it is working harder than necessary and guessing more than they should.
The repair, itemised
Flame sensors, ignitors, pressure switches, capacitors and condensate pumps are inexpensive parts and quick jobs. Inducer motors, blower motors and gas valves are a step up. Heat exchangers are in their own category and change the conversation entirely.
Watch forA lump sum with no named part. You cannot compare quotes or judge value without knowing what is being replaced and what the part costs.
The heat exchanger question
A cracked heat exchanger lets combustion gases into the air you breathe, and it is the one finding that ends a furnace rather than repairing it. On an older unit, replacing it usually costs a large fraction of a new furnace.
Watch forA cracked heat exchanger announced verbally with no evidence. This is the highest-value diagnosis a contractor can make, which is exactly why you should ask for a camera photograph or a combustion reading supporting it before you accept a replacement quote - and get a second opinion.
Repair or replace, on age
Under about twelve years, a normal parts repair is easy to justify. Past eighteen, a major repair on a unit with no warranty left rarely is.
Watch forA same-visit replacement quote produced before any part has been tested. On a cold day, under pressure, that is an effective sales technique and a poor way to spend several thousand dollars.
What does furnace repair cost?
Most furnace repairs come in between $150 and $650. Flame sensors, ignitors and condensate blockages sit at the bottom; inducer motors, blower motors and gas valves at the top. A cracked heat exchanger is not really a repair - it is a replacement decision.
See the full furnace repair cost guide - by material, size and region →
If the number is the problem, here are the ways people pay for it →
HomeMatchup does not perform furnace repair. We match you with licensed local hvac pros for furnace repair. Free for homeowners — Get matched free · how we verify pros.
When furnace repair is the wrong call
When saying yes is the wrong move:
- You have not checked the filter or the thermostat batteries. A blocked filter overheats the exchanger and trips the limit switch, and a dead thermostat battery looks exactly like a dead furnace. Both are free to rule out and both account for a real share of call-outs.
- The heat exchanger is cracked and the furnace is over fifteen years old. The part cost plus the labour to get to it typically approaches a new furnace with a full warranty. Price the replacement before authorising the repair.
- You are replacing the AC this year anyway. The evaporator coil sits on the furnace. Patching the furnace now and doing both in six months means paying twice for the same access and giving up a properly matched system.
- The complaint is uneven heat, not a breakdown. Nothing has failed. Rooms that never get warm are a duct sizing, leakage or balancing problem, and a repair technician swapping parts will not change it.
- The unit is oversized and short-cycling. No part is faulty. It fires, satisfies the thermostat quickly, and shuts off - and no repair fixes a sizing decision that was made when it was installed.
- This is the third repair in two winters on an old furnace. Add up what you have already spent. A run of individually reasonable repairs frequently totals more than the replacement would have.
Can I do this myself?
Reasonable to DIY
- Replacing the air filter - the single most common cause of a furnace tripping its high limit and shutting down
- Confirming the thermostat is on heat, has working batteries, and is set above room temperature
- Checking the furnace door switch is fully closed - the unit will not run with the panel loose
- Resetting the breaker once, and checking the separate on/off switch that looks like a light switch near the unit
- Clearing snow, leaves or a bird's nest away from the exterior intake and exhaust terminations on a high-efficiency furnace
- Testing your carbon monoxide alarms and replacing them if they are past their printed expiry date
Call a licensed pro
- Anything involving the gas valve, the burners or the manifold
- Cleaning or replacing the flame sensor - routine for a technician, but it sits directly in the burner assembly
- Any diagnosis or repair of the heat exchanger, the flue or the venting
- Replacing the inducer or blower motor, or any wiring on the control board
- Any furnace that has tripped a CO alarm, or that a technician has red-tagged and shut off
What to check when hiring for furnace repair
- Will you inspect the heat exchanger and check the flue as part of the diagnosis? On combustion equipment this comes before the parts diagnosis. A company that treats it as optional is not doing furnace work properly.
- What fault code is the control board showing? A specific, checkable answer. It also tells you whether the technician read the furnace or started from a guess.
- If you are telling me the heat exchanger is cracked, can I see the evidence? The highest-value diagnosis in the trade and the one most worth a second opinion. Ask for a camera image or a combustion reading, not a description.
- What is the diagnostic fee, and is it credited if I go ahead? Standard practice varies. Ask before booking, and ask whether evening or weekend rates apply.
- Are you licensed for gas work in this state? Gas fitting is separately licensed in many jurisdictions. It is a fair question and an easy one for a legitimate company to answer.
- Will you take a combustion reading after the repair? It confirms the appliance is burning cleanly and venting properly once the work is done - the difference between fixed and merely running.
Furnace Repair near you
Local pro matching for furnace repair is rolling out by metro. Enter your ZIP and we'll match you with local pros in your area today.
Guides & terms
Sources
Sources last checked
- Central Air Conditioner and Heat Pump SEER2 / HSPF2 StandardsU.S. Department of EnergyRetrieved 2026-08-04Federal efficiency metrics that appear on HVAC equipment labels and quotes.
- ENERGY STAR Certified Air Conditioners & Heat PumpsENERGY STAR / EPARetrieved 2026-08-04Independent efficiency criteria homeowners can check against a bid.
Sources for figures and claims on this page.
Get furnace repair quotes
Matched hvac pros. Free for homeowners.
Free · No obligation