Furnace Keeps Shutting Off? Why It Short Cycles and What to Do

What does it mean when your furnace shuts off so quickly?

How long the furnace runs before it shuts off points to the cause.

What should I check first when my furnace keeps shutting off?

Check the filter, the vents and returns, the thermostat and the outside vent pipes first. None of these need tools, and none involve opening the furnace.

1. Air filter. A clogged filter is the most common cause of overheating. If you can't see light through it, replace it. Fallbrook's dry, dusty days and Santa Ana winds clog filters faster than the package suggests.
2. Supply vents and return grilles. Open closed vents and move furniture, rugs or boxes off the return grilles. Closing too many vents raises the pressure and the temperature inside the furnace.
3. Thermostat. Replace the batteries, and make sure the thermostat isn't in direct sun, near a lamp or close to a supply vent. A warm thermostat thinks the house is heated and shuts the furnace off early.
4. Outside vent pipes (high-efficiency furnaces). If your furnace has white PVC pipes going outside, check that they aren't blocked by leaves, a bird nest or overgrown plants.
5. Water near the furnace. Water around the base or in the drain pan usually means a clogged condensate line, which can shut the furnace down.
6. Light code. Count the blinks on the status light through the small window on the furnace door and write them down. It tells a technician where to look first.

Don't keep resetting the furnace by switching the power off and on. If it locks out again after these checks, leave it off and call.

Why does my furnace keep turning on and off?

Short cycling is usually the furnace protecting itself from overheating, a flame it can't confirm, or a venting problem. These are the causes from most to least likely.

1. Dirty flame sensor

The flame sensor is a thin metal rod in front of the burners. Over time it gets coated, can't detect the flame, and the control board shuts off the gas within seconds. This is one of the most common furnace repairs and is usually quick to fix. It is also why some furnaces blow cold air between attempts; see why a furnace blows cold air.

2. Overheating from restricted airflow

A clogged filter, closed vents, blocked returns or a weak blower motor trap heat inside the furnace. The high-limit switch shuts the burners off to protect the heat exchanger, the blower cools things down, and the cycle repeats. A limit switch that trips often is a warning sign, not just a nuisance.

3. Pressure switch, vent or condensate problem

Before the burners light, the inducer motor pulls air through the vent and a pressure switch confirms it. A blocked vent, a cracked hose, a failing inducer or a clogged condensate drain on a high-efficiency furnace stops the start-up.

4. Thermostat issues

Low batteries, loose wires or a thermostat placed in a warm spot can turn the furnace off early. Older homes in Downtown Fallbrook sometimes have thermostats on walls that get afternoon sun or sit near a supply register.

5. Furnace too large for the home

An oversized furnace reaches the setpoint fast and shuts off before air reaches the far rooms. This is common after an older replacement was sized by the label rather than by the home. Custom homes on larger lots in Morro Hills and Gird Valley, with long duct runs, are a frequent example.

6. Failing control board or electrical fault

A worn control board, loose connections or a weak transformer can cause random shutdowns. The blink code helps narrow it down.

When should I stop and call for furnace repair?

Call right away if you smell gas or burning, if the furnace locks out, or if the checks above didn't stop the shutoffs.

• You smell gas or rotten eggs. Leave the house and call 911 from outside, then your gas company or propane supplier.
• You smell burning, see scorch marks, or the breaker trips.
• The furnace locks out with a blinking light after several tries.
• The limit switch keeps tripping even with a new filter and open vents.
• There is water under a high-efficiency furnace.

Do I need a new furnace if it keeps shutting off?

Usually not. A flame sensor, filter, pressure switch, drain or thermostat problem is a repair. Replacement comes into the picture when the heat exchanger is cracked, when the furnace is badly oversized, or when an older unit keeps needing major parts. The furnace replacement page explains how to compare the two. If the furnace runs but dust is the problem, a furnace cleaning may help.

Furnace keeps shutting off: FAQs