Homeowner Guide

Why Won't My Furnace Ignite?

A furnace that goes through its startup sequence but never actually lights — or lights briefly and shuts back off — is one of the most common calls we get from Ohio homeowners once the weather turns cold. Most causes are mechanical and routine. A few are worth treating as a safety issue first.

Where Ignition Failures Usually Start

A gas furnace has to clear several checks before it's allowed to light. Ignition failure almost always traces back to one of these:

• Flame sensor buildup — by far the most common cause; a coated or corroded sensor can't detect a flame, so the control board cuts gas flow as a safety measure even though the burner briefly lit
• Worn-out igniter — hot surface igniters and spark igniters have a limited lifespan and eventually stop producing enough heat or spark to start combustion
• Pressure switch not closing — this switch confirms the draft inducer motor is venting exhaust properly before gas is allowed to flow; a stuck or misaligned switch blocks ignition entirely
• Vent or intake blockage — exterior terminations can get packed with debris, ice, or nesting material, which throws off the draft the pressure switch is checking for
• Something simpler — the furnace's power switch (it looks like a household light switch) got bumped off, or the gas valve was closed for another reason

Mechanical Fault vs. Safety Issue

A furnace that clicks a few times, tries to light, and locks out is doing what it's designed to do — protecting against an unsafe ignition, not creating one. That's a repair call, not an emergency. It becomes a safety issue if you notice a rotten-egg smell (the odorant added to natural gas specifically so leaks are noticeable), hear hissing near a gas line, or a carbon monoxide detector goes off. In any of those situations, get everyone out of the house, don't flip switches or light anything on the way out, and call your gas utility's emergency line from outside before you call anyone else, including us.