The furnace lights, then shuts off

Two patterns, two causes

Lights, then goes out within seconds. The flame sensor cannot prove there is a flame, so the gas valve closes. A thin coating on the sensor rod is the most common reason. After three or four tries the board locks out.

Runs a few minutes, shuts off, starts again. That is overheating. A clogged filter, closed registers or a weak blower starves the furnace of air, and the limit switch shuts the burners off until it cools. An oversized furnace can do the same thing on a mild day.

On an oil furnace, the cad cell plays the flame sensor's part. A dirty cell or a sputtering nozzle makes the burner start and quit, and the primary control eventually locks out with the red button up.

What you can check

• Pull the filter and hold it to a light. If no light comes through, replace it.
• Make sure supply registers and the return grille are not blocked by furniture or closed.
• Note how long each run lasts. A count with a watch is useful to the technician.

What not to do

Do not keep resetting the power or the oil burner. Each reset adds another failed start. On oil, repeated resets can leave unburned fuel in the chamber.

What a technician checks

A technician reads the board's fault code, measures the flame signal, checks the pressure switch and the temperature rise across the heat exchanger, and inspects the flue. If the temperature rise is high, airflow is the problem; if the flame signal is weak, the sensor is.

What to say when you call

• Gas or oil, and roughly how old the furnace is
• How long it runs before it shuts off
• The number of flashes on the board light
• When the filter was last changed
• Address and ZIP code