Start With the Thermostat
The thermostat is the most common culprit and the easiest to rule out. Confirm it is set to HEAT, the temperature setting is above the current room reading, and the display is lit. Dead batteries blank the screen silently. If the thermostat is programmable, check that it is not in a hold or vacation mode overriding your setting.
Check Power to the Furnace
Furnaces have a dedicated power switch, usually a standard light switch on or near the unit, sometimes with a red cover. Make sure it is on. Then check the breaker panel for a tripped furnace breaker. Reset it once. If it trips again immediately, stop and call, because something is shorting.
Listen to What It Tries to Do
Stand near the furnace and have someone raise the thermostat. You should hear the inducer fan start, then clicking or a glow as ignition attempts, then the burners light. Where the sequence stops tells the technician exactly where to look. Note what you hear before you call.
When to Stop Troubleshooting
If you smell gas, hear loud banging, or see the carbon monoxide detector alarming, stop and call for help rather than continuing to troubleshoot. For everything else on this checklist, a failed check just means it is time for a professional diagnosis. Call (571) 561-3454 and describe what you found.
Questions About Why Won't My Furnace Turn On
Why does my furnace click but not start?
The igniter may be trying and failing, or the flame sensor may not be proving flame. Both are common and repairable.
Can a dirty filter stop the furnace?
A severely clogged filter can trip the high-limit switch, shutting the furnace down. Check and replace the filter as a first step.
How long should I wait before calling?
Run through the thermostat, power, and filter checks once. If the furnace still will not start, call rather than waiting.
