Furnace Not Starting? 8 Things to Check Before Calling (2026)

Check the Thermostat

Start here every time. Confirm the thermostat is set to HEAT (not COOL or FAN only), the temperature setpoint is higher than the current room temperature, and the batteries are not dead. Smart thermostat with a blank screen? The batteries or C-wire power supply may have failed. Replace batteries first.

Check the Air Filter

A completely blocked air filter is the single most common cause of furnace shutdowns. A clogged filter starves the furnace of return air, causing it to overheat and trip the high-limit safety switch. The furnace shuts off for safety, cools down, starts again, then shuts off again in a repeating cycle. Pull the filter out and hold it up to light. If you cannot see through it, replace it immediately.

Check the Circuit Breaker

Furnaces run on 120V electrical power in addition to natural gas. If the furnace breaker has tripped, the control board has no power and the furnace cannot operate. Go to your electrical panel and look for the furnace breaker. If it is in the middle position (tripped), push it fully to OFF first, then back to ON. If it trips again immediately, there is a wiring issue - call EcoFrost.

Check the Furnace Power Switch

The furnace has a dedicated power switch that looks exactly like a standard light switch, usually mounted on the furnace itself or on the wall nearby. It is often accidentally switched off by someone who did not realize what it was. Confirm it is in the ON position.

Check the Gas Supply

Locate the manual gas shut-off valve on the gas line leading to the furnace. The valve is open when the handle is parallel to the pipe and closed when it is perpendicular. Also confirm your home has gas supply - check if your gas stove works (if applicable). If no gas appliances work, there may be a supply interruption; call Enbridge at 1-866-763-5427.

Read the Error Code

On most gas furnaces, a LED on the control board blinks a pattern to indicate the fault. Count the blinks (a pause separates each sequence). Flash codes differ by brand and model, so the same number of blinks can mean different faults on different furnaces. Read the code chart on the inside of the furnace door, which lists what each flash code means for your specific model.

Check the Exhaust Vent Outside

High-efficiency furnaces (96%+ AFUE) exhaust through PVC pipes that terminate at the exterior of your home, typically near the foundation. In Ontario winters, frost and ice can completely block the vent termination. Go outside and check both the exhaust (white PVC, may have steam) and intake pipes for ice or debris blockage. Clear any obstruction with your hand or warm water - never use a sharp tool on the PVC.

Power-Cycle the Furnace Once

If the furnace tried to start and failed, it may be in ignition lockout. A lockout usually clears if you turn the furnace power switch off for 30 seconds and back on, or turn the thermostat off and back on. Do this once only. A manual-reset button on the burner compartment is usually a flame rollout safety switch, and it must not be reset by the homeowner. If it has tripped, something unsafe caused it: call a licensed gas technician. If the furnace does not start after one power cycle, call for service.