If the furnace keeps turning off before the house warms up, a dirty filter or blocked vent could be restricting airflow. Check the accessible filter and make sure vents are clear. If it keeps happening, note how long it runs and call for service instead of repeatedly resetting it.

A little cool air at startup can be normal. If it keeps blowing cold, check that the thermostat is set to Heat, the temperature is set above the room temperature, and the fan is on Auto. Check the filter if it’s easy to access. If there is still no heat, book an inspection.

A code tells you what the furnace detected, not necessarily which part failed. Codes vary by model. If the light or display is visible without opening the unit, take a photo or short video. Include it and the furnace model when you request service.

A light tick can come from ducts warming up. If you hear a new, loud bang at startup or a continuing rattle, turn the furnace off and arrange an inspection. If you smell gas, don’t touch the thermostat or any switches. Leave immediately and call Enbridge Gas or 911 from a safe location.