Furnace Starts Then Shuts Off? Here's the #1 Cause (and the 10-Minute Fix)
You hear the furnace fire up, feel a brief pulse of warm air, then everything goes quiet again. Five minutes later, it repeats. Before you call an HVAC technician for an emergency winter visit, check one thing first. It fixes this problem more often than anything else — and it costs nothing.
Why This Happens
Your gas furnace has a critical safety device called a flame sensor — a thin metal rod that sits directly in the path of the burner flame. Its only job is to confirm that the gas actually ignited. When the thermostat calls for heat, the furnace opens the gas valve and lights the burner. The flame sensor detects the heat and tells the control board: "Flame is present, it's safe to keep the gas flowing."
Over a heating season, that metal rod gets coated with carbon, dust, and combustion residue. Once the coating gets thick enough, the sensor can't detect the flame reliably — even when the burner is clearly lit. The control board, not receiving confirmation, shuts off the gas valve as a safety precaution. The flame goes out, the blower keeps running for a minute to clear residual gas, then the whole cycle starts over. This "short-cycling" pattern — on for 10 to 30 seconds, off for a few minutes, then on again — is the classic signature of a dirty flame sensor. It's responsible for more no-heat service calls in winter than any other single part, and cleaning it takes 10 minutes with a common household item.
Signs You're Dealing With This
- The furnace ignites, burns for a few seconds, then shuts off before the house warms up
- The cycle repeats every 3 to 5 minutes, but the blower never runs long enough to heat the house
- You see the burners light through a viewing window, but they go out almost immediately
- The thermostat is set normally, but the house temperature keeps dropping
- The furnace worked fine last season, and the filter is clean
- You haven't had annual maintenance in over a year
The Fix, Step by Step
- Turn off the power to the furnace. Flip the breaker at your electrical panel, or switch off the furnace disconnect switch (usually a metal box mounted on the side of the unit). The flame sensor is part of the live ignition system, so this step is non-negotiable.
- Remove the furnace access panel. Most panels lift up and off, or are held by a few screws along the top or bottom edge. Set the panel aside carefully.
- Locate the flame sensor. It's a thin metal rod — usually straight, sometimes L-shaped or bent at 45 degrees — mounted in front of or beside the burner assembly. It has a single wire connected to the end and is held in place by one screw. It looks like a small spark plug or a thick piece of wire.
- Remove the screw holding the sensor. Use a 1/4-inch nut driver or a small Phillips-head screwdriver. Most are hand-tight. Gently pull the sensor out of its bracket. It may be stiff from heat cycling — wiggle it slightly, but don't bend it.
- Disconnect the wire. The wire connector usually pulls straight off with gentle pressure. Note which direction it faces, or take a photo for reference.
- Clean the metal rod with fine steel wool or a soft wire brush. This is the entire fix. Scrub the rod from end to end until the metal shines clean and silver. Do not use sandpaper — it leaves scratches that trap debris faster. Do not use soap, water, or cleaning chemicals — they leave residue that interferes with flame detection.
- Wipe the rod with a clean, dry cloth to remove any steel wool fibers, then reinstall it in the same position. Tighten the mounting screw firmly but don't over-torque — the bracket is thin metal.
- Reconnect the wire, replace the access panel, and turn the power back on. Set the thermostat to "heat" and raise the temperature 5 degrees above the current room temp. The furnace should fire up, the burners should stay lit, and warm air should flow continuously within a minute or two.
When to Call a Professional Instead
If you've cleaned the flame sensor, confirmed the filter is new, and the furnace still short-cycles or won't stay lit, the issue is likely a failing flame sensor (cleaning no longer works), a faulty control board, a bad gas valve, or a cracked heat exchanger. A technician can test the sensor's microamp output and inspect the heat exchanger for cracks with a camera. If you see rust, soot, or water pooling under the furnace, call a professional — these indicate combustion or venting problems that are dangerous to ignore.
Quick Questions
How often should I clean the flame sensor?
Once a year, ideally before the heating season starts. It's a 5-minute job that prevents the most common winter furnace failure. Many HVAC technicians include it in annual maintenance, but doing it yourself saves the service call.
Can I use a dollar bill or sandpaper instead of steel wool?
A crisp dollar bill works in an emergency — it's mildly abrasive and won't scratch the rod. Sandpaper is too aggressive and creates grooves that collect carbon faster. Fine steel wool (grade #000 or #0000) is the gold standard.
Why does the sensor get dirty so fast?
Dust, pet hair, and airborne particles get pulled into the combustion air intake and burn onto the sensor. Homes with pets, recent construction, or a dirty basement tend to coat the sensor faster. A clean furnace filter helps, but won't eliminate it entirely.
My furnace doesn't have a flame sensor — it has a pilot light. Is this the same?
No. Older standing-pilot furnaces use a thermocouple instead of a flame sensor. If your pilot won't stay lit, the thermocouple is the likely culprit — a different part, but also a cheap and easy DIY replacement. Check your furnace manual or the data plate to confirm which ignition system you have.
Still short-cycling after trying this? Check out our guide on furnace pressure switches and vent blockages — the next most common cause.