The Burner Lights, Then Quits
The inducer motor starts. A click. The burners light with a soft whoosh, and you see blue flame through the little window. Then three seconds later it goes out. Your furnace shuts off after 3 seconds, tries again, and does the same thing. After a few tries it goes quiet and the house keeps getting colder.
That pattern matters, because it rules a lot out. Your gas is on. Your ignitor works. The gas valve opens. The furnace is doing almost everything right and then deciding, on purpose, to shut the gas off. It's not dying. It's refusing to keep going because one check didn't pass.
If your ignitor glows and the burners never light at all, that's a different problem with a different list of causes. This post is for the furnace that lights and then quits.
What the Furnace Checks in Those Few Seconds
Once the burners light, the control board gives the flame a short window to prove it's there. Usually that's a few seconds. The proof comes from the flame sensor, a thin metal rod that sits in the flame. The board sends a small AC voltage to that rod. Flame conducts a tiny current from the rod back to the burner, and the burner is grounded to the furnace cabinet. The board reads that current in microamps. Enough current, the board keeps the gas valve open. Not enough, it closes the valve.
That's the whole reason your furnace shuts off after a few seconds. The board can't tell the flame is there, so it assumes it isn't. That's a safety feature, not a failure.
Most boards retry three times, then lock out. Many lock out for an hour before trying again, which is why the furnace sometimes comes back on by itself later and fails the same way. While it's locked out, the board is usually flashing a code. Our guide to reading furnace blinking light codes covers how to count them and what the common ones mean. On this problem you'll often see a flame-sense or ignition-lockout code.
You Cleaned the Flame Sensor and It Still Shuts Down
Every post on this topic says the same thing: clean the flame sensor. It's good advice. A coated sensor is the most common cause, and a fine abrasive pad fixes it in five minutes. But plenty of people do it, button the furnace back up, and hear the exact same three-second shutdown. One Ruud owner wrote it word for word: "I cleaned flame sensor, still shutting down." Here's what we check next.
The ground path
The flame signal only works if the current has a clean path back to the board. That path runs through the burner assembly, the cabinet, and the ground wire to the board. A rusty screw, a corroded ground terminal, or a loose green wire breaks it. The sensor can be spotless and the board still reads nothing.
The sensor wire and the porcelain
The wire from the sensor to the board can crack inside its insulation or loosen at the spade connector. The white porcelain around the rod can crack too, which lets the signal leak to the bracket instead of going through the flame.
The flame itself
If the flame lifts off the burner, burns lazy and yellow, or doesn't wrap around the rod, the sensor isn't in good flame. Dirty burners, rust flakes, a spider nest in a burner tube over the summer, or low gas pressure can all do this. Low pressure is a meter reading at the gas valve. That's a technician job. Don't adjust the gas valve yourself.
Polarity
Flame sensing depends on the hot and neutral being wired the right way. If they're swapped, at the furnace switch or at an outlet the furnace plugs into, many boards can't read flame at all.
The board
Sometimes it's the control board. We don't start there, because boards aren't cheap and they're the most commonly misdiagnosed part on this call. We measure the flame signal in microamps first. If the signal is strong and the board still drops the flame, then it's the board.
When the Pressure Switch Is the One Shutting It Off
There's a second way to get a short run and a shutdown, and it gets mixed up with flame sensing. Before the burners light, the inducer motor pulls air through the heat exchanger and out the vent. A pressure switch proves that draft. If the switch opens after ignition, the board shuts the gas off just the same.
On a high-efficiency furnace with white PVC pipes, the usual suspects are a clogged condensate trap, a kinked or water-filled pressure switch hose, a blocked intake or exhaust pipe outside, or an inducer that's wearing out. Water sitting in the drain lines can make the switch flutter open and closed.
The difference matters. A flame-sense problem shows a different blink code than a pressure-switch problem. Reading the code before anyone touches a part saves you from paying for a flame sensor you didn't need.
One thing we won't do is jumper out a pressure switch or a limit switch to keep the heat on. Those switches are there to stop exhaust from backing into the house. If someone offers to bypass one "just for tonight," say no.
Why Cincinnati Furnaces Do This in the First Cold Weeks
The furnace sat all summer. Dust settled on the burners, the sensor picked up a film, and on high-efficiency units the condensate trap dried out. Then it runs for the first time and fails on the second or third cycle.
Freeze-thaw weather adds its own version. Side-wall PVC vents can collect frost or ice at the end of the pipe on a cold night, especially low vents near snow, shrubs, or a deck. Enough of that and the pressure switch can't prove draft.
A lot of the older east-side housing stock, in Mariemont, Mount Washington, and Madeira, has basements that stay damp. Damp means rust on burner screws and ground connections, and that's the ground-path problem from above. Wiring updated in pieces over the decades is where swapped polarity comes from.
What You Can Check Before You Call
A few checks are safe and free. Start here before you spend anything.
- Count the seconds. Lights and quits in under ten seconds points to flame sense or draft. Runs several minutes then shuts off is a different problem, closer to a system that keeps short cycling.
- Read the blink code through the little window in the lower door before you cut power. Power cycling clears it.
- Look at the PVC pipes outside. Clear snow, ice, leaves, or a nest from the ends of the intake and exhaust. Don't cut or modify the pipes.
- Check the condensate drain for standing water or a hose that's pinched.
- If you're comfortable, clean the flame sensor once with a fine abrasive pad, not coarse sandpaper. Power off first. Don't touch the ignitor.
What not to touch: the gas valve, the pressure switch, the wiring on the board, and anything behind the burner area when it's been running. If you smell gas at any point, stop, leave the house, and call Duke Energy's gas emergency line from outside.
If the sensor was the whole problem, you're done for the price of a scrubbing pad. If you cleaned it and the furnace still shuts off after a few seconds, the fix is testing, not more parts. A good visit looks like this: the tech reads the code, puts a meter on the flame signal in microamps, checks the ground and polarity, checks the pressure switch and the drain, and shows you the number that failed before quoting a part. Most of these repairs are a wire, a ground, a cleaning, or a drain line. Our furnace and heating repair visits are built around that order.
Renew Mechanical's diagnostic is $89, and it's credited toward a repair you approve on the same visit. Our technicians are salaried, not on commission, so nobody makes more by selling you a board you don't need. We cover Anderson Township, Mount Washington, Newtown, Mariemont, Madeira, Loveland, Milford, and Amelia. If your furnace lights and quits, call us at (513) 951-8539.