Homeowner Guide

Why Won't My Furnace Ignite?

A furnace that runs through its startup cycle but never lights — or lights briefly and then shuts back off — is one of the most common reasons Michigan homeowners call us during the heating season. Most causes are mechanical, not dangerous, but a few are worth treating as a safety issue first.

What Usually Stops Ignition

Modern furnaces go through a startup sequence, and ignition failure usually traces back to one point in that sequence:

• Dirty or failing flame sensor — the most common cause; the sensor can't confirm a flame is present, so the control board shuts the gas valve for safety
• Failed igniter — the hot surface igniter or spark igniter wears out over years of use and simply stops producing enough heat or spark to light the burners
• Pressure switch stuck or misaligned — this switch confirms the draft inducer is pulling exhaust properly before allowing gas flow; if it doesn't close, the furnace won't proceed to ignition
• Clogged flue or blocked intake/exhaust vent — snow, ice, or debris around an exterior vent termination is common in Michigan winters and can prevent the draft the pressure switch is checking for
• Closed gas valve or tripped power switch — worth checking before anything else; the furnace switch looks like a household light switch and is sometimes bumped off accidentally

When It's a Safety Issue, Not Just a Repair

If the furnace repeatedly tries to ignite and shuts off (multiple clicks followed by silence), that's a mechanical fault the control board is protecting against — not dangerous on its own. But treat it differently if you smell gas (often described as a rotten-egg odor, since natural gas has an odorant added specifically so it's noticeable), hear a hissing sound near the gas line, or a carbon monoxide detector activates. In any of those cases, leave the house, do not operate switches or light anything on your way out, and call your gas utility's emergency line from outside before calling anyone else, including us. Once the gas company confirms it's safe, we can diagnose and repair the actual mechanical cause.