Why Does My Furnace Keep Shutting Off? 9 Causes and Fixes

Quick answer: A furnace that keeps shutting off is usually overheating from a clogged filter or blocked vents, losing its flame signal because of a dirty flame sensor, failing a pressure switch or limit switch check, venting through a blocked pipe, or getting bad signals from the thermostat. Check the filter, vents, thermostat and outdoor pipes first. Don't keep resetting it. If it still shuts off, book furnace repair. If a CO alarm sounds or you smell gas, leave and call 911 or your gas utility from outside.

A furnace that won't stay on leaves your home swinging between chilly and warm, and it wears out parts quickly. The good news is that its timing often points right to the cause. Here's how to use it.

Start with the timing

See also short cycling explained.

1. Clogged air filter

This is the most common cause, and the easiest fix. Poor airflow traps heat in the furnace until the limit switch shuts off the burners. After the unit cools, it starts again and repeats. Replace the filter and see if it runs normally. See the air filter guide.

2. Dirty flame sensor

The flame sensor proves that burners are lit. Over time it gets a coating that weakens the signal, so the board shuts off the gas, even with a healthy flame. Typically the burners run for a few seconds, then quit, then retry. A technician can clean and test it. See dirty flame sensor.

3. Tripped limit switch

The limit switch shuts the burners off when the heat exchanger gets too hot. Common reasons include blocked airflow, closed vents, a weak blower or undersized ducts. A limit switch that keeps tripping is a warning, because overheating stresses the heat exchanger. See cracked heat exchanger signs.

4. Blocked vent or intake

High-efficiency furnaces use plastic pipes for intake and exhaust. Snow, ice, leaves or nests can block them, and the pressure switch stops the furnace from running. After a storm, check the pipes outside. For older furnaces, chimney blockages can be dangerous, so call for service if you suspect one.

5. Pressure switch or inducer problems

Before the burners light, the inducer fan draws air through the furnace, and the pressure switch confirms it's working. A failed inducer, cracked hose, wet line or blocked vent can stop the sequence. Sometimes it starts fine and then trips when conditions change. See furnace won't ignite.

6. Blocked condensate drain

High-efficiency furnaces produce water that must drain away. If the trap or line is clogged or frozen, a safety switch shuts the furnace off. You may also notice gurgling or water near the unit. See frozen condensate lines and furnace leaking water.

7. Thermostat problems

A thermostat can cause short cycles if it has weak batteries, loose wiring, poor placement near a heat source or a sunny wall, or incorrect settings. Try fresh batteries and make sure nothing warm sits near it. See thermostat troubleshooting.

8. Oversized furnace

A furnace that's too big heats the house quickly, shuts off and restarts soon after. It may never run long enough to work efficiently. If the problem has always been there, sizing may be the cause. A load calculation will tell you. See how to size a system.

9. Failing control board or blower

Control boards and blower motors can fail intermittently. Heat can make the fault appear only after the unit has been running for a while. The fault code history helps a technician find the problem. See furnace blower not working.

Safe checks you can do

1. Replace the filter if it's dirty.
2. Open supply vents and clear returns of furniture and rugs.
3. Check the thermostat settings and batteries.
4. Look outside for blocked intake and exhaust pipes.
5. Check the door panel to make sure it's seated.
6. Read the fault light: count the blinks, then use the chart inside the door.
7. Reset once: turn the power off for 30 seconds, then back on.

Stop there. Don't remove or bypass switches, and don't clean internal parts unless you're trained.

When to stop and call right away

• Carbon monoxide alarm: leave the house and call 911. See carbon monoxide safety.
• Gas smell: leave and call 911 or your gas utility from outside.
• Burning or electrical smell: turn off the furnace at the service switch.
• Soot, scorch marks or flames outside the burner area
• A lockout that returns after one reset
• No heat in freezing weather: see no heat and frozen pipes.

Why you shouldn't ignore it

Short cycling costs more than a chilly room:

• Higher gas bills: startups are the least efficient part of the cycle
• Faster wear on igniters, motors and relays
• Heat exchanger stress from repeated overheating
• Uneven temperatures from room to room
• A bigger failure on the coldest night

How we find and fix it

1. Check the thermostat signal and read the fault history
2. Watch a full heating cycle from start to finish
3. Measure temperature rise and airflow
4. Test the flame sensor, igniter, pressure and limit switches
5. Check gas pressure, venting and condensate
6. Inspect the heat exchanger and test for carbon monoxide
7. Explain the cause and give you an upfront price before any work begins

See what affects furnace repair cost.

Repair or replace?

Most of these fixes are straightforward repairs. It's worth comparing replacement if your furnace is around 15 to 20 years old, has a cracked heat exchanger or needs repeated repairs, or if it has always been the wrong size. See signs you need a new furnace and furnace replacement in New Jersey. New systems qualify for $500 off (terms apply).

How to prevent it

• Book a yearly heating tune-up: an $89 tune-up is available (terms apply)
• Change filters on schedule
• Keep vents, returns and outdoor pipes clear
• Have condensate lines checked before winter

The Prime Comfort Club covers one heating and one AC tune-up a year for $19/month (terms apply).

What to tell us when you book

Mention how long the furnace runs before it quits, whether the blower keeps going, the number of blinks on the fault light and when you last changed the filter. Those details often narrow the problem down before we arrive.

A short video of a full cycle, from startup to shutoff, can also save diagnostic time, because intermittent faults don't always appear on demand.

Why homeowners choose us

We're family-owned and veteran-owned, and our own licensed team does the work, never subcontractors. You get same day service, a 24/7 emergency line, upfront pricing with no hidden fees, and service on every brand.

Same day furnace repair near you

We repair every brand of furnace across all 153 towns in Ocean, Monmouth, Burlington and Middlesex counties, including Brick, Jackson, Woodbridge and Marlton.

Furnace won't stay on? Book online or call (732) 779-7106, day or night. New customers save $50 on their first repair (terms apply).