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A repair technician finds out why the furnace is not heating properly, replaces or adjusts the faulty part, and checks that it burns cleanly and vents safely before leaving. Gas, propane and oil furnaces combine fuel, flame, electricity and exhaust in one cabinet, so any work on the burner, the gas valve, the venting or the wiring must be done by a technician licensed for that fuel in your province.
Before you book, a few safe checks can sometimes save a service call, and knowing the warning signs of carbon monoxide can protect your family. Below you will find the signs that point to a repair, what you can safely check yourself, what a repair visit involves and how to tell when fixing an old furnace no longer makes sense.
Signs your furnace needs repair
- The furnace runs, but the air coming out of the registers is cool or barely warm.
- It turns on and off in short bursts, or it won't start at all.
- You hear new noises, such as a loud bang when it lights, grinding from the blower or a high-pitched whine.
- Through the viewing window, the burner flame looks yellow and wavering instead of a steady blue.
- You notice soot around the furnace or the registers, or a burning smell that doesn't fade after the first heating days.
- Water is collecting under a high-efficiency furnace.
- Some rooms never reach the temperature set on the thermostat.
- A status light on the furnace is blinking an error code.
Safe furnace checks to try before calling for repair
Some no-heat calls come down to a setting or a switch, and these checks involve no tools and no contact with gas parts. Stop there: don't remove burner covers, touch the gas valve, bypass a safety switch or keep resetting a furnace that locks itself out. Those steps belong to a licensed technician.
- Make sure the thermostat is set to heat, that the set temperature is above the room temperature and that its batteries are fresh if it uses them.
- Check the furnace breaker in the electrical panel and the service switch near the furnace, which looks like a light switch and is easy to turn off by mistake.
- Pull out the filter and replace it if it is grey and clogged, since restricted airflow can make the furnace overheat and shut itself off.
- Make sure the furnace's front panel is closed properly, because many furnaces won't run while the door safety switch is open.
- Outside, clear snow, ice or frost from the furnace's exhaust and air intake pipes.
- Open the supply registers and move furniture or rugs off the return grilles.
Carbon monoxide safety when your furnace isn't working right
A furnace that burns fuel produces carbon monoxide, a gas with no smell, taste or colour that only a carbon monoxide alarm can detect. A cracked heat exchanger, a blocked vent or chimney, or poor combustion can let it into the house. Health Canada lists tiredness, headaches, flu-like symptoms and shortness of breath as signs of low-level exposure, and dizziness, chest pain, poor vision and difficulty thinking at higher levels.
If the alarm sounds, leave the home immediately without looking for the source, and call 9-1-1, your fire department or emergency services once you are outside. Don't restart the furnace until a qualified technician has found and fixed the cause. Health Canada considers hallways outside sleeping areas the most important place for a carbon monoxide alarm.
What happens during a furnace repair visit
The technician starts by asking what you noticed and when, then reads any error code on the furnace's control board. They test the thermostat signal, the igniter, the flame sensor, the pressure switch and the inducer and blower motors, and inspect the filter, the condensate drain and the venting.
Once the faulty part is found, they explain the repair and whether the part is on the truck; parts for older or less common furnaces sometimes have to be ordered. After the repair, they run the furnace through complete heating cycles, check combustion and gas pressure where it applies, and confirm that exhaust is leaving the house as it should.
Furnace repair or replacement: how to decide
A single failed part on a furnace that is otherwise in good shape is usually worth fixing. The decision gets harder when breakdowns repeat, when the part is expensive or hard to find, or when the technician finds a problem with the heat exchanger.
- Lean toward repair when the furnace is fairly young, has been maintained and the problem is a common wear part such as an igniter, a flame sensor or a capacitor.
- Consider replacement when the heat exchanger is cracked or badly corroded, since it can let combustion gases into the home.
- Consider replacement when breakdowns come back winter after winter.
- Consider replacement when parts for your model are no longer made.
- Consider replacement if you already plan to change your heating, for example to a heat pump, and the repair would only buy a short time.
- Ask the technician to put their findings in writing so you can compare them with a second opinion.
Avoiding a furnace breakdown in the middle of winter
Furnaces tend to fail when they work hardest, during long cold spells. A fall inspection gives a technician the chance to spot a weak igniter, a worn blower motor or a dirty flame sensor before it leaves you without heat. Keep a spare filter on hand and note the furnace's model and serial number, which speeds up ordering parts.
If the heat goes out while you wait for a technician, close the doors of unused rooms, let faucets on outside walls drip slightly to protect the pipes, and use only certified space heaters kept well away from curtains and bedding. Never try to heat the house with an oven, a barbecue or a fuel-powered generator, since they can fill it with carbon monoxide.
Furnace Repair: frequently asked questions
Why is my furnace running but blowing cold air?
Check first that the thermostat fan is set to auto rather than on, since the on setting moves air even when the burners are off. If that isn't it, the burners may not be lighting or staying lit, often because of a dirty flame sensor, a failed igniter, a blocked vent or a clogged condensate drain, which a technician should diagnose.
Why does my furnace keep turning on and off?
Short cycling can come from a clogged filter that makes the furnace overheat, a thermostat placed near a heat source, a dirty flame sensor or a furnace that is too big for the house. Replace the filter first; if the cycling continues, have it checked, because constant starts wear out parts faster.
Can I reset my furnace myself?
Turning the service switch off and back on once is generally fine and sometimes clears a lockout. If the furnace locks out again, stop: repeated lockouts mean a safety device is doing its job, and the cause needs a licensed technician. Never bypass a safety switch or work on the gas controls.
My carbon monoxide alarm went off. Can I keep using the furnace?
No. Get everyone out, call emergency services from outside and let them clear the home. Leave the furnace off until a qualified technician has inspected it, including the heat exchanger and the venting, and confirmed that it is safe to use.
Is a broken furnace an emergency in winter?
It can be. With no heat in freezing weather, pipes can freeze and burst and the house can become unsafe for babies, older people and anyone with health problems. Many HVAC companies offer after-hours service in winter; say clearly that you have no heat when you call.
How long does a furnace repair take?
Many common repairs are finished in a single visit when the technician carries the part. It takes longer when a part for an older or less common furnace has to be ordered, or when the problem only shows up under certain conditions and needs more testing.
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Written by the LokalMatch editorial team. Reviewed by Zubair Afzal on September 10, 2026. How we write and check our guides