HVAC · Service

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.

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In short

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.

Triggers

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 →

The job

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 for: A 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 for: A 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 for: A 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 for: A 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 for: A 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.

Money

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 →

Straight answer

When furnace repair is the wrong call

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

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.

Straight answer

Can I do this myself?

A furnace burns fuel inside your home. If a carbon monoxide alarm sounds, if you smell gas, or if the burner flame is yellow and sooty, do not troubleshoot — leave the house and call your gas utility or the fire service. No repair on this page is worth staying inside for.

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

Before you sign

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.

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Related work

Often done at the same time

Furnace Installation

When repair stops making sense

Emergency HVAC Repair

No heat, after hours

HVAC Maintenance

The annual visit that prevents most of this

AC Repair

The cooling side

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Go deeper

Guides & terms

Guides

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Terms on this page

AFUE

Full hvac glossary →

FAQ

Furnace Repair — questions people ask

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What should I do if my carbon monoxide alarm goes off?

Get everyone out of the house first, including pets, and call your gas utility's emergency line or the fire service from outside. Do not open windows to "air it out" and go back in — that can mask the reading the responders need. Do not restart the furnace. Once the property is cleared, a technician needs to inspect the heat exchanger and the venting before the appliance goes back into service. Treat a single alarm as real, even if it stops on its own.

Why does my furnace turn on then shut off after a minute?

The most common cause by a distance is a dirty flame sensor. It is a thin metal rod in the burner flame that proves combustion is happening; when a film builds on it the signal drops, the control board decides the flame is not lit, and it shuts the gas off as a safety response. Cleaning it is a quick and inexpensive job. A blocked condensate drain on a high-efficiency furnace, or a blocked flue, produce a similar pattern.

Is it safe to keep using a furnace with a cracked heat exchanger?

No. The heat exchanger is the wall between the combustion gases and the air circulating through your house. A crack in it can put carbon monoxide into your living space, which is why technicians are expected to shut the appliance down and tag it rather than leave the decision to you. If you doubt the diagnosis, get a second opinion the same day rather than continuing to run it in the meantime.

Is it worth repairing an old furnace?

It depends on the repair, not just the age. A couple of hundred dollars on an ignitor to get an eighteen-year-old furnace through another winter is perfectly rational. A large sum on a gas valve or a blower motor on the same unit usually is not, because the warranty is gone, efficiency has fallen well behind current equipment, and the next failure is likely within a season or two.