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 technicians 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, and the order below runs from most to least common — worth checking in that order rather than assuming the worst first:

• 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
• Control board fault — the least common cause but the hardest to self-diagnose, since it can produce almost any symptom on this list intermittently
• 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. Outside of those specific warning signs, a furnace that simply won't light is a mechanical problem worth scheduling promptly during cold weather but not one that requires evacuating the house.

• Rotten-egg smell, hissing near a gas line, or a CO detector alarm — treat as an emergency: leave the house, don't flip switches, call the gas utility from outside.
• Clicking, trying to light, and locking out with no unusual smell or alarm — a mechanical repair call, not an emergency, though worth scheduling promptly in cold weather.
• Check the power switch and thermostat batteries first — both are quick to rule out and require no technical knowledge before calling for service.
• A furnace that ignites but then shuts off within seconds points more specifically at the flame sensor than at the igniter, which helps a technician diagnose faster on arrival.

This content is reviewed by the Ohio Furnace Repair editorial team and informed by licensed-contractor sourcing, manufacturer documentation, and established industry standards for furnace repair. Read our editorial standards.