Furnace Won’t Stay Running: Causes, Troubleshooting, and Fixes

The article explains why a furnace won’t stay running, how to diagnose common problems, and practical fixes homeowners can try before calling a technician. It focuses on safety, clear troubleshooting steps, and preventive maintenance to keep heating systems reliable.

Symptom	Probable Cause	Quick Fix
Furnace Starts Then Stops	Dirty Flame Sensor, Overheat Limit Switch	Clean sensor, Replace filter
Short Cycling	Incorrect Thermostat Settings, Oversized Furnace	Check thermostat, Call HVAC pro
Runs A Few Seconds	Pilot Or Ignition Failure	Inspect pilot/igniter, Replace part

How Furnaces Normally Operate

A typical gas or electric furnace follows a sequence: thermostat calls for heat, ignition or electric elements engage, burners heat the heat exchanger, blower circulates air, and the system runs until the thermostat is satisfied. A furnace that won’t stay running interrupts that sequence, often repeating a start-stop cycle known as short cycling.

Common Causes When A Furnace Won’t Stay Running

Several mechanical, electrical, and maintenance-related issues cause a furnace to stop prematurely. Identifying the correct cause narrows the repair and prevents repeated failures.

Clogged Air Filter And Restricted Airflow

A dirty or clogged filter restricts airflow, causing the furnace to overheat and trip safety switches. Filters should be checked monthly during heavy use. Restricted airflow can also result from closed vents or blocked return grilles.

Dirty Flame Sensor Or Ignition Problems

A flame sensor detects burner ignition. When it is fouled, the control board may shut the burners off shortly after ignition to prevent unburned gas accumulation. Electronic igniters and pilot assemblies can also fail, causing repeated shutdowns.

Faulty Limit Switch Or Overheat Condition

The high-limit switch stops the furnace if the heat exchanger gets too hot. Frequent trips indicate overheating, often caused by poor airflow, a failing blower motor, or a restricted heat exchanger.

Thermostat Issues

A malfunctioning thermostat, incorrect settings, or improper wiring can cause the furnace to cycle off prematurely. Thermostat placement near drafts or heat sources can also provide false readings.

Blower Motor Or Capacitor Problems

If the blower motor or its capacitor fails, the system may run briefly but shut down as safety systems detect inadequate air movement, leading to overheating and shutdown.

Short Cycling Due To Oversized Furnace

An oversized furnace heats the space too quickly then shuts off, causing many short cycles. This reduces comfort, increases wear, and raises energy costs.

Gas Supply Or Pressure Issues

Insufficient gas pressure or interrupted supply can cause burners to extinguish. Valve, regulator, or piping problems require professional diagnosis for safety reasons.

Electrical Problems And Control Board Failures

Loose connections, failing relays, or a defective control board can interrupt the furnace’s run cycle. These issues often present as intermittent operation or complete shutdowns.

Step-By-Step Troubleshooting Checklist

The following steps help diagnose why a furnace won’t stay running. Many tasks are safe for homeowners; some require a licensed HVAC technician.

1. Check The Thermostat: Verify the thermostat is set to heat, at a temperature above room temperature, and on the correct mode (heat vs. fan). Replace batteries if applicable.
2. Inspect And Replace Air Filters: Remove and examine the air filter. Replace disposable filters or clean reusable ones. A clean filter improves airflow and reduces limit trips.
3. Listen For Short Cycling: Note how long the furnace runs before stopping. Runs shorter than five minutes often indicate short cycling.
4. Examine The Flame Sensor: For gas furnaces, a flame sensor can be cleaned with fine emery cloth. Turn off power and gas before accessing the sensor.
5. Look For Error Codes: Many modern furnaces flash LED codes on the control board indicating specific faults. Consult the unit’s manual for interpretation.
6. Check Vents And Registers: Ensure supply registers and return grilles are open and unobstructed. Verify attic or crawlspace vents used for combustion air are clear.
7. Inspect The Blower: Turn off power and look for excessive dust, debris, or a seized motor. A failing motor may hum, make grinding noises, or not run consistently.
8. Verify Gas Supply: Confirm other gas appliances work. If gas is interrupted or low pressure is suspected, contact the gas utility and avoid DIY gas repairs.
9. Reset The System: In some cases, turning off power for 30 seconds and restoring it resets control board errors. Recheck operation after reset.

Safety Precautions Before Any DIY Work

Working on furnaces can be hazardous. Homeowners should turn off power at the furnace switch and shut the gas valve before opening panels. If smell of gas or other unsafe conditions exist, leave the home and call the gas company or emergency services.

When To Call A Professional HVAC Technician

Call a licensed technician if troubleshooting doesn’t resolve the issue, if gas components or control boards require replacement, or if any electrical work is necessary. Professionals provide proper diagnostics, safe gas handling, and guaranteed repairs.

Signs A Pro Is Needed

• Persistent short cycling after basic maintenance
• Gas smells, yellow burner flames, or suspected carbon monoxide risk
• Repeated high-limit trips or overheating
• Blower motor failure or unusual electrical smells
• System age over 15 years with frequent failures

Typical Repairs And Cost Ranges

Repair costs vary by region and furnace type. The following ranges offer general guidance for U.S. homeowners.

Repair	Typical Cost Range	Notes
Air Filter Replacement	$10–$50	DIY or service call adds labor
Flame Sensor Cleaning/Replacement	$75–$250	Cleaning low cost; new sensor higher
Thermostat Replacement	$100–$350	Smart thermostats higher
Blower Motor or Capacitor	$300–$900	Depends on motor type and labor
Control Board Replacement	$300–$800	Includes diagnostics and labor
Gas Valve Or Igniter	$150–$500	Professional gas service required

Preventive Maintenance To Keep A Furnace Running Smoothly

Regular maintenance reduces the chance a furnace won’t stay running. Simple seasonal tasks extend equipment life and improve comfort.

• Replace or clean filters every 1–3 months during heating season.
• Schedule an annual HVAC tune-up to inspect burners, heat exchanger, ignition, and safety controls.
• Keep vents and returns clear and dust-free. Vacuum registers and relocate furniture blocking airflow.
• Install a programmable or smart thermostat to reduce short cycles and optimize run time.
• Ensure combustion air pathways are clear for older furnaces requiring outdoor air.

Special Considerations For Different Furnace Types

Gas Furnaces

Gas furnaces are prone to flame sensor contamination, pilot/igniter issues, and gas valve failures. Carbon monoxide risk exists with cracked heat exchangers; annual inspections are critical.

Electric Furnaces

Electric furnaces rely on heating elements and a blower. Failures often involve the blower motor, limit switches, or control relays. Electric models have no combustion risk but can pose electrical hazards.

High-Efficiency Condensing Furnaces

Condensing furnaces include additional components like secondary heat exchangers and condensate drains. Blocked drains or pump failures can cause shutdowns and require specialist attention.

Frequently Asked Questions

What Is Short Cycling And Why Is It Bad?

Short cycling occurs when a furnace runs for very short periods and repeatedly turns off. It increases wear on components, wastes energy, and reduces comfort. Identifying the root cause is essential.

Can A Dirty Filter Cause The Furnace To Turn Off?

Yes. A dirty filter restricts airflow, causing the furnace to overheat and trip safety switches. Replacing the filter often restores normal operation.

How Long Should A Heating Cycle Last?

Cycle length depends on furnace size, home insulation, and outdoor temperature. Typical cycles run 10–20 minutes. Much shorter cycles suggest a problem.

Is It Safe To Clean The Flame Sensor?

Cleaning a flame sensor is a common homeowner task if comfortable working with the furnace; always shut power and gas before opening the panel. If unsure, hire a technician.

Checklist For Homeowners Before Calling For Service

Before calling an HVAC tech, gather information to speed diagnosis: model and age of furnace, symptom timeline, any recent maintenance, error codes or LED flashes, and whether other gas appliances are affected. Provide this info to improve repair accuracy.

Key Takeaway: A furnace that won’t stay running often results from airflow restrictions, ignition or sensor faults, thermostat issues, or mechanical failures. Start with basic checks—thermostat, filter, vents—then escalate to professional service for gas, electrical, or complex component repairs.