Furnace Keeps Igniting Then Shutting Off After A Few Seconds – Causes and Fixes

Why Does a Furnace Ignite Then Shut Off After a Few Seconds?

When a furnace keeps igniting then shutting off after a few seconds, the flame sensor is almost always the cause — not the igniter. The sensor is a thin metal rod near the igniter that proves to the control board a flame actually exists. A healthy sensor sends back a signal in the 3 to 6 microamp DC range, and once carbon buildup drops that reading below about 1 microamp, the board cuts the gas rather than risk running with an unproven flame — even though you can clearly see fire through the access panel. My first instinct was always to blame the igniter, since it’s the part doing the visible work of sparking, but I learned that’s backwards — the igniter already proved itself the moment it lit the burners. The real problem sits downstream, in that thin sensing rod losing its ability to prove the flame is still there.

The sensor picks up a dull gray-white coating of carbon and oxidation after a season or two of use — a dirty sensor looks completely normal until you know what to check for (trust me on this one). That coating doesn’t block the flame, it just insulates the tiny microamp signal, the same way corrosion on a battery terminal kills a connection even though the wire itself is fine. After nineteen years of Air Force aircraft maintenance, I’ve learned that safety systems like this tend to fail quietly instead of loudly, and a furnace flame sensor is no exception. Dirty filters and furnaces that haven’t had the sensor cleaned in years both speed up the buildup.

The wrong move I see constantly is homeowners grabbing a screwdriver and repositioning the igniter instead, since it’s the visible part. Don’t — bending it can crack the fragile element inside, turning a free fix into a $40-80 part replacement.

A Stuck Pressure Switch Cuts Gas Almost Instantly

The pressure switch is a small plastic switch with a rubber diaphragm that confirms the draft inducer motor is actually pulling exhaust before the gas valve opens. If it drops out mid-cycle, the burners shut down almost immediately — within a second or two — rather than after several seconds like a flame sensor problem does. Suspect this cause anytime shutoff happens nearly the instant the burners light, especially after a windstorm or when the vent pipe outside gets partially blocked by nesting insects, snow, or dryer lint. Cleaning or replacing the switch runs $20-40 for the part, and it’s usually just a two-wire disconnect and one or two screws — 15 minutes if it’s easy to reach, longer if it’s buried behind the blower housing.

You’ll usually hear the draft inducer motor humming along fine when this is the issue. I always check the switch’s rubber tubing before reaching for a multimeter — if it’s cracked, kinked, or full of condensation, that’s your answer without even needing one.

Timing It: What the Number of Seconds Before Shutoff Tells You

Here’s something I don’t see covered anywhere else on this topic, and it’s the fastest way to narrow things down before you touch a screwdriver: count how many seconds pass between ignition and shutoff.

• Instant shutoff, within a second or two: usually points to the pressure switch or a wiring issue in the sensing circuit, not a dirty sensor.
• Three to five seconds: this is the classic flame sensor signature. The board’s trial-for-ignition window is typically 4-7 seconds, and a weak sensor signal fails right at the edge of it.
• Runs 30-60 seconds or longer, then quits: this points more toward a marginal sensor reading, a loose wire connector gone intermittent from vibration, or a limit switch tripping as airflow can’t keep up.

None of this replaces testing the sensor, but it saves you from pulling the wrong part first. You’ve gotta count the seconds before grabbing a screwdriver — it’s saved me more wasted trips than I can count. If your furnace keeps igniting then shutting off inside that 3-5 second window every cycle, I’d start with the sensor before anything else.

How to Test and Clean the Flame Sensor Yourself

Testing before replacing saves a needless parts purchase more often than you’d expect — many “failed” sensors read just fine once cleaned, since the real corrosion problem is usually on the wire connector, not the rod itself. I test before I replace, every time (I check this one every single time, no exceptions).

1. Kill the power. Shut off the furnace at the switch on the unit or at the breaker — don’t just turn the thermostat down.
2. Find the sensor. It’s the thin metal rod near the igniter, usually held in with a single screw.
3. Remove and inspect it. If the rod is coated in gray-white buildup, that’s your answer.
4. Clean it gently. Fine steel wool, emery cloth, or 400-600 grit sandpaper works — you just need the coating off, not bare shiny metal.
5. Reinstall and restore power. Watch a full ignition cycle and see if it holds.
6. If you own a multimeter that reads microamps DC, connect it in series with the sensor wire during the trial-for-ignition window and confirm you’re seeing that 3-6 microamp range.

This step-by-step matches the process shown in this video, worth a quick watch before you pull anything apart:

I see this same sensor design constantly on Goodman and Amana furnaces from the mid-2000s through about 2015, usually held in with that single hex-head screw. If your furnace runs on a standing pilot instead of a hot surface igniter, the failure looks different — check out how to fix a pilot light on a gas furnace that won’t stay lit, since that’s almost always a thermocouple issue, not a flame sensor. For general cleaning technique, Repair Clinic’s flame sensor cleaning guide covers the same steps with good photos. If the rod’s cracked or pitted beyond cleaning, Repair Clinic’s flame sensor parts page has most models for $10-20.

Overheating and Airflow Problems Trip the Limit Switch

A furnace that runs a full 30-60 seconds or longer before shutting off is kinda a different animal than an instant cutoff, and I look at airflow first. A clogged filter, closed vents, or a dusty blower wheel all restrict airflow across the heat exchanger. The furnace overheats, the limit switch trips as a safety measure, and the burners shut down even though the flame was proving itself the whole time. You’ll often smell a faint hot-dust odor right before it happens.

This one’s free to fix if it’s just a dirty filter — pull it, hold it up to a light, and if you can’t see light through it, replace it. Filters run $10-20 depending on size and rating. If a dusty blower wheel is the real cause, that’s closer to an hour-long job, and I’d call a pro if you’ve never had a furnace apart before.

The Rare Case: A Cracking Igniter or a Failing Control Board

Every once in a while the igniter itself is the problem, but not because it won’t spark. A hairline crack in a hot surface igniter can let it glow and even light the burners, but the crack changes its resistance enough that it draws less current than the board expects. Some boards use that current draw as a secondary check, and a marginal igniter can trigger a lockout even with a good sensor. I’d rank it well behind the sensor, pressure switch, and limit switch, but it fools people because the igniter looks like it’s doing its job.

A failing control board is rarer still, and it’s the diagnosis of last resort after the sensor tests good, the pressure switch cycles properly, and the limit switch isn’t tripping from airflow. I wouldn’t touch the control board until every cheaper cause was ruled out first. Trane’s own troubleshooting guide lists the same handful of causes furnace manufacturers see most — flame sensing, gas supply, venting, and airflow — before control boards even enter the conversation, which matches what I’ve seen in the field.

How to Stop This From Coming Back

A few habits keep this from becoming a yearly headache:

1. Clean the flame sensor once a year, ideally right before heating season starts. It takes about ten minutes and it’s the single best thing you can do to avoid a mid-January lockout.
2. Change the filter every 1-3 months depending on how dusty your house runs — that’s the same airflow restriction that trips the limit switch.
3. Check the vent pipe termination once a year. Make sure it isn’t blocked by leaves, snow, or a wasp nest. I’ve traced more than a few pressure switch failures straight back to that.

If you’ve ever dealt with a dehumidifier that runs constantly but never actually collects water, or a washer that seems stuck for no clear reason, it’s the same category of headache — the unit’s clearly doing something, just not the thing it’s supposed to do. I broke down that troubleshooting logic for the dehumidifier here, and the same logic applies to a washer stuck on lock cycle: check the cheap sensor before you assume the expensive motor or board is dead.

Common Questions About a Furnace That Keeps Shutting Off

What does it mean if my furnace keeps igniting then shutting off?

It almost always means the control board can’t confirm a stable flame, even though the flame is there. Check the sensor first — nine times out of ten that’s the fix.

Can I clean a flame sensor with just sandpaper?

Yep, fine-grit sandpaper or emery cloth works fine. Just don’t sand it down to bare shiny metal; a light pass to remove the coating is all it needs.

How much does it cost to replace a flame sensor?

The part itself runs $10-20. A service call for the same fix usually lands around $150-250, which is a rough price for such a simple job.

Is a furnace that keeps shutting off dangerous?

The lockout itself is the safety system working correctly — it’s designed to stop gas flow rather than risk an unproven flame. It’s not dangerous by itself, but don’t ignore it for weeks, since a bigger issue like a cracked heat exchanger can eventually cause the same symptom.

Why does my furnace run fine for a minute then shut off?

That longer run time usually points to airflow — a dirty filter or a limit switch tripping from overheating — rather than the sensor itself. I’d check the filter before touching anything else.

I still think this is one of the more satisfying furnace fixes out there — cheap, fast, and it makes you look like you know what you’re doing. I’d rather see homeowners pull that sensor and give it a wipe than pay for a full service call on a ten-minute fix.