Furnace Won Start — Complete Troubleshooting Guide
Published: July 23, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC
Check Thermostat First
Ensure the thermostat is set to HEAT mode (not COOL or OFF). Set the temperature 5°C above the current room temperature. Replace batteries if the display is blank or fading. If the thermostat is hardwired, check the circuit breaker for the furnace. A simple thermostat issue is the most common cause of a furnace not starting.
Check Power and Gas
Check the furnace circuit breaker in the main panel — toggle it fully OFF and ON. Check the furnace power switch (looks like a light switch on the furnace or wall near it) — must be in the ON position. Check the gas supply valve on the gas line — must be parallel to the pipe (ON). If the gas was recently turned off, air may need to be purged from the line.
Check Safety Switches
Check the furnace blower door — if the panel is not securely closed, a safety interlock switch prevents the furnace from running. Press the panel firmly to engage the switch. Check the condensate float switch — if the condensate drain is clogged, a safety switch shuts down the furnace. Clear the drain to reset.