Furnace Not Working? 8 Things to Check
Before you call for a repair, try these 8 troubleshooting steps. Many furnace problems have simple fixes that any homeowner can handle. If none of these work, we offer 24/7 emergency repair across the GTA.
Safety First
If you smell gas (rotten egg odour), leave the house immediately. Do not turn on lights, use phones, or create any sparks. Call Enbridge Gas emergencies at 1-866-763-5427 from outside. If your carbon monoxide detector is alarming, evacuate and call 911. These are emergencies that require immediate professional response.
8 Troubleshooting Steps
Work through these steps in order. Each one addresses a common cause of furnace problems, starting with the easiest and most likely fixes.
Check Your Thermostat
This is the most common cause of furnace "failures" and the easiest to fix. Make sure your thermostat is set to HEAT mode (not COOL or OFF), the temperature is set higher than the current room temperature, and the fan is set to AUTO rather than ON.
Pro tip: If your thermostat is battery-powered, try replacing the batteries. A blank or unresponsive thermostat screen often just means dead batteries.
Check the Air Filter
A clogged air filter is the number one cause of furnace problems. When the filter is blocked, airflow is restricted and the furnace can overheat and shut down as a safety measure. Pull out the filter and hold it up to a light. If you cannot see through it, replace it.
Pro tip: Standard 1-inch filters should be replaced every 1-3 months. If you have pets or allergies, check monthly. A clean filter prevents most common furnace issues.
Check the Circuit Breaker
Even gas furnaces need electricity to run the blower motor, ignitor, and control board. Go to your electrical panel and look for the breaker labelled "Furnace" or "HVAC." If it is tripped (in the middle position), switch it fully OFF, then back ON.
Pro tip: If the breaker trips again immediately or repeatedly, do not keep resetting it. This indicates an electrical issue that needs professional diagnosis.
Check the Gas Valve
Your furnace has a gas shutoff valve, usually located on the gas pipe leading to the unit. Make sure it is in the ON position. The handle should be parallel to the pipe (in line with it). If the handle is perpendicular (at a right angle), the gas is off.
Pro tip: If you recently had other gas work done in your home, the technician may have shut off the furnace gas supply. This is a quick check that is often overlooked.
Check the Pilot Light or Ignitor
Older furnaces have a standing pilot light that can blow out. If you have an older unit, look through the small viewing window on the front panel for a flame. Modern furnaces use an electronic ignitor, which does not have a visible pilot but can fail over time. If you hear the furnace clicking but it does not fire up, the ignitor may need replacement.
Pro tip: Relighting a pilot light is straightforward on older units — follow the instructions printed on the furnace panel. Do not attempt to relight it if you smell gas.
Check the Condensate Drain Line
High-efficiency furnaces (90%+ AFUE) produce condensation that drains through a small plastic tube. If this line becomes clogged with algae, mold, or debris, the furnace will shut down to prevent water damage. Look for standing water near the base of the furnace or a full condensate tray.
Pro tip: You can often clear a clogged condensate line by pouring a mixture of warm water and vinegar through it. If it is severely blocked, a wet/dry vacuum on the outdoor end can clear it.
Check the Condensate Pump
If your furnace is in a basement, it likely has a condensate pump to move water up and out. These small pumps can fail, get clogged, or simply lose power. If the pump reservoir is full of water and the pump is not running, the float switch will shut down your furnace as a safety measure.
Pro tip: Test the pump by pouring water into the reservoir. If the pump does not activate, check that it is plugged in. If it is plugged in and still will not run, the pump needs replacement.
Try the Reset Button
Most furnaces have a reset button, usually a red or yellow button on the front panel or near the blower motor. If your furnace locked out due to a temporary issue (power surge, brief gas interruption), pressing the reset button once may resolve it.
Pro tip: Press the reset button once and wait 30 minutes. Do not press it repeatedly. If the furnace does not stay running after one reset, the problem requires professional diagnosis.
When to Call a Professional
If you have worked through all 8 steps and your furnace still is not working, or if you encounter any of the following situations, it is time to call for professional service.
TSSA-certified technicians. Upfront pricing. No hidden fees. Serving all of Toronto and the GTA.
Furnace Troubleshooting FAQs
Why is my furnace running but blowing cold air?
Why does my furnace keep turning on and off?
Is it dangerous to run a furnace that is not working properly?
How much does an emergency furnace repair cost in Toronto?
Should I call for furnace repair or just replace it?
Still Not Working? We Are Here 24/7
If your furnace is not cooperating after trying these steps, call Megacity HVAC. We offer 24/7 emergency furnace repair across Toronto and the GTA with upfront pricing and no surprise charges.