Gas Furnace Turning Off? Why It Lights and Then Shuts Down

What you're seeing

• Burners light, then go out within a few seconds.
• The furnace runs for a few minutes, shuts down, then tries again.
• The inducer runs but the burners never light.
• The board LED is flashing a code.

How to track it down

Work through these in order, or follow along in the video above. Each step rules something out before you spend money on parts.

1. Read the flash code

   Most furnaces flash a code on the control board. Count the flashes and match it to the chart on the door. It points you straight at the safety that tripped.

2. Out in a few seconds: flame sensor

   If the burners light and drop out within a few seconds, the board isn't proving flame. A dirty flame sensor is the most common cause, along with a bad ground, reversed polarity, or low gas pressure.

3. Out after a few minutes: overheating

   If it runs for several minutes, the high limit is probably opening. Check the filter, closed registers, the blower, and a dirty evaporator coil sitting on top of the furnace.

4. Won't light at all: pressure switch

   If the inducer runs but it never tries to light, check the pressure switch, its hose, and the venting. A blocked flue, a cracked hose, or water in the collector box will keep it from closing.

5. Check the thermostat too

   A thermostat mounted near a supply register, or set with a very short cycle, can shut the furnace off early and look like a furnace problem.

If you're a homeowner

Changing the filter and making sure registers are open is safe for anyone. Cleaning a flame sensor is a common fix, but it's inside a gas appliance, so if you're not comfortable, call a pro. If you ever smell gas, leave the house and call your gas company from outside.

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