Why Is My Furnace Not Kicking On?
A furnace that will not kick on is most commonly caused by a failed ignitor (the part that lights the burners), a tripped safety switch, a thermostat issue, or a clogged filter that triggered the high-limit switch. The ignitor is the most common part to fail on modern furnaces — they wear out every 5-7 years and cost $150-$250 to replace.
THE DETAILS
A failed ignitor is the number one reason furnaces will not kick on. You may hear the draft inducer motor start, but the burners never light. Ignitors wear out and are a normal maintenance item.
The high-limit safety switch shuts the furnace down if it overheats. A clogged filter is the most common cause — the furnace overheats because it cannot move enough air.
A failed flame sensor causes the furnace to light briefly (1-3 seconds) then shut off. The sensor is dirty or worn and does not detect the flame, so the system shuts down as a safety precaution.
The draft inducer motor must run before the furnace will fire. If it has failed, the furnace will not start at all because the control board requires confirmation that exhaust venting is working.
The thermostat may not be sending the heat signal. Try turning it to a higher temperature to force a call for heat.
🔧 WHEN TO CALL A PRO
If your furnace will not start after checking the thermostat and filter, call a tech. Ignitor, flame sensor, and draft inducer problems all need professional repair.