Burners light. Warm air for six or seven minutes. Then the flame goes out, the fan keeps blowing for a while, and the house never quite reaches the number on the thermostat. When your furnace keeps shutting off like this, it is almost never random. Something inside the cabinet is deciding to stop the burners, and on a modern furnace that decision usually comes from one of a handful of safety controls.

Those controls are there to protect you and the equipment, so the goal isn’t to get around them. It is to work out which one is tripping and why. Once you know that, the fix is often small.

If you want the general picture of why frequent on-off cycles wear out heating and cooling equipment, our post on short cycling covers it. This article sticks to the furnace itself.

Get a Stopwatch Before You Get a Screwdriver

The most useful thing you can do first costs nothing. Stand near the furnace or a vent through two or three full cycles and write down:

• how long the burners stay lit before they go out
• whether the blower keeps running after the flame stops
• whether the furnace starts again by itself, and how long it waits
• the pattern of the status light on the control board, if your furnace door has a peephole

A flame that dies within seconds, a flame that runs several minutes and then cuts out, and a furnace that runs a normal cycle but far too often are three different problems. That record also gives a technician a head start, which keeps a repair visit shorter.

The Flame Sensor: Out Within Seconds

If the burners light and go out after only a few seconds, usually several times in a row before the furnace gives up, the control board probably can't confirm the flame is there. The flame sensor is the part that tells it. When the sensor's metal rod picks up a coating, the signal gets too weak, and the board shuts the gas valve to be safe.

Furnaces that sit unused through a long Las Vegas summer are prime candidates, because the first weeks of heating season are when a coated sensor finally gets tested. After a set number of failed tries, most control boards stop trying and go into lockout. Some will try again on their own after a while; others wait for the power to be cycled.

Cleaning a flame sensor is routine work, but it sits right beside the burners and gas valve. A technician will also check that the burner flame is hitting the sensor properly and that the furnace is grounded correctly, since both can cause the same symptom on a clean sensor.

The High-Limit Switch: Out After Several Minutes, Fan Still Running

When the burners run for a few minutes, shut off, and the blower carries on pushing air that quickly turns cool, the high-limit switch is the usual suspect. It cuts the burners when the area around the heat exchanger gets too hot. The furnace then cools down, the switch resets itself, and the cycle repeats.

Overheating almost always means too little air is moving across the heat exchanger. The filter is the first suspect, and if yours hasn't been changed since summer, start there. How to check and replace it is laid out in our article on a heater blowing cold air.

When a fresh filter doesn't help, look beyond it. Closed supply registers in several rooms, a return grille behind a couch, or a sagging duct run in the attic can all choke airflow. Inside the furnace, a blower wheel caked with dust or a weak blower motor does the same thing. Those last two need a technician, as does any limit switch that keeps tripping with good airflow, because the switch itself can fail.

Never jumper or bypass a limit switch. It exists to stop the heat exchanger from overheating, and repeated overheating is one of the ways heat exchanger metal ends up cracked.

The Pressure Switch: Out Before or Right After Ignition

Before any gas flows, a small inducer fan starts up to clear the exhaust path, and a pressure switch confirms that the draft is there. If the switch doesn't see enough draft, the furnace stops, sometimes before the burners light and sometimes moments after.

On high-efficiency furnaces, which vent through plastic pipes out a side wall or roof, check the pipe ends from outside. Debris, a wasp nest or a bird can partly block them. Clearing something you can see at the opening is fine. Anything deeper in the pipe, a worn inducer motor, or a cracked hose to the switch is a technician's job.

The Condensate Switch: Water With Nowhere to Go

High-efficiency furnaces make water as they burn gas, and it drains away through a small line, often to a pump. If that line clogs or the pump fails, a float switch shuts the furnace off before water spills.

The clue is often visible: water around the base of the furnace, a full pump reservoir, or a damp drain pan. In homes where the furnace sits in the attic, a clogged condensate line is worth taking seriously, since an overflow there ends up in the ceiling below. Algae and mineral buildup inside the line are common culprits.

The Rollout Switch: The One You Should Not Reset

Rollout switches sit near the burners and trip if flames curl out of the burner area instead of being drawn into the heat exchanger. That points to a blocked flue, a blocked heat exchanger or a cracked one. Many rollout switches have a small reset button, and pressing it gets the furnace running again without fixing whatever pushed the flames out. A tripped rollout switch is a reason to leave the furnace off and book a repair. If a carbon monoxide alarm is also sounding, get everyone outside and call 911.

When Nothing Is Broken

Sometimes every part works and the furnace still runs in short bursts. That usually comes down to the thermostat or the size of the furnace.

Test the thermostat's reading. Set a simple indoor thermometer on a shelf in a bedroom and compare it with the thermostat after a cycle ends. If the thermostat is satisfied while the bedroom is several degrees cooler, it is sensing a warm pocket, often from a nearby register, a lamp, afternoon sun or the kitchen, and switching the furnace off early.

Consider the furnace's size. A furnace with far more capacity than the house needs reaches the setpoint fast, shuts off and starts again as soon as the temperature dips. In the Valley, where winters are mild, a furnace chosen mainly to pair with the air conditioner can end up with more heating capacity than the house really needs. A technician can compare the furnace's output with the home's heating load and tell you whether a thermostat change, a furnace setting or a different system makes more sense.

Reset Once, Then Call

Restarting the furnace once to see what happens is reasonable, as long as the status light pattern is already on paper, because many control boards forget it once the power goes off. If it shuts off again in the same way, stop. Repeated resets keep putting the furnace back into whatever condition tripped the switch, and that wears out ignitors and stresses the heat exchanger.

If your furnace won't start at all, rather than starting and stopping, start with our guide for a furnace not turning on instead.

Getting the Furnace to Finish Its Cycle

Most furnaces that keep shutting off need one small repair, not a new system. When you're ready, book a same-day heating repair visit with Dr. Cool online or by calling (702) 873-1800, and have your stopwatch notes handy. You'll know the price before any work begins. An annual heating maintenance visit each fall is the easiest way to catch a dirty sensor or a slow drain before it interrupts a cold night.