Furnace Won't Ignite

When your thermostat calls for heat and the inducer motor spins up but the burner never lights, your furnace is failing partway through its ignition sequence. This is one of the most common winter no-heat calls we get — and the fix is usually cheap if you catch it before you're already three hours into a freeze. Here's how to narrow down the cause.

Most Likely Causes

1. Dirty flame sensor (most common by far)

The flame sensor is a small metal rod that sits in the burner flame and confirms to the control board that a flame is actually present. If it's coated with soot or oxidation — which happens naturally over 12-18 months of use — it can't detect the flame, so the control board shuts the gas valve off within seconds. Symptoms: burner lights briefly (you might see a puff of blue flame) then immediately cuts off, and the cycle repeats 2-3 times before the control board locks out. A $10 flame sensor cleaning fixes 60% of no-heat calls we run in a winter.

2. Failed hot-surface igniter

Modern gas furnaces use a silicon carbide or silicon nitride igniter that glows white-hot to ignite the gas — no pilot light. These fail on average every 5-7 years. Symptoms: no glow, no ignition attempt, control board typically flashes an error code. You can see the igniter through an access panel on most furnaces — if it doesn't glow orange when heat is calling, that's your problem. Replacement is $220-$340 including labor.

3. Blocked condensate drain (high-efficiency furnaces only)

If you have a 90%+ AFUE condensing furnace, it produces acidic condensate that has to drain out. When that drain line clogs (which happens if it hasn't been cleaned in a couple years), a safety pressure switch trips and shuts the furnace down. Symptoms: no ignition attempt at all, control board flashes a pressure switch code. Fix is to clear the condensate trap and drain line.

4. Failed gas valve or low gas pressure

If the igniter glows properly but no gas arrives at the burner, either the gas valve isn't opening on command or your inlet gas pressure is too low. Common after utility work in your neighborhood, or after propane runs low if you're on LP. Gas valve replacements run $420-$640. Low pressure requires the utility or a plumber.

5. Tripped high-limit safety switch

If the furnace overheated on a previous cycle (usually from a dirty filter restricting airflow), the high-limit switch may have latched open. Some reset themselves after cool-down; others must be manually reset or replaced. Symptoms: inducer runs but no ignition attempt, sometimes intermittent — works when cold, fails after a full heating cycle.

6. Rollout safety switch tripped

If a burner has ever produced 'flame rollout' (flames rolling out the front of the burner cabinet, usually from a blocked heat exchanger passage or venting problem), the rollout switch pops and locks the furnace out. This is a genuine safety issue — don't just reset it and hope. The underlying cause needs to be found. Call us.

Try These Steps Before You Call

• Verify the thermostat is calling for heat (set 5°F above room temp, switch to heat mode).
• Check the air filter — a clogged filter causes overheating that trips safety switches. Replace if dirty.
• Check the furnace power switch (looks like a light switch on or near the furnace) — make sure it's on.
• Check the breaker labeled 'furnace' or 'air handler' — flip fully off then back on.
• Look for a flashing LED on the furnace control board through the sight glass — count the blinks; the code identifies the failure. Most furnaces have a code chart on the door panel.
• For high-efficiency furnaces, check the condensate drain — pour a cup of vinegar down the drain access if you have one.
• If you have a pilot-light older furnace (rare now), see our pilot light won't stay lit guide.

Other Common Heating Problems

See also: AC problems