Why a Furnace Won’t Stay on and How to Fix It

The following article explains common reasons a furnace won’t stay on, step-by-step troubleshooting, safety considerations, and when to call a professional. It is designed for U.S. homeowners seeking practical guidance to diagnose and resolve intermittent furnace shutdowns while maintaining safety and efficiency.

Common Cause	Symptoms	Quick Fix
Thermostat Issues	Furnace cycles off quickly, display errors	Check settings, replace batteries, recalibrate or swap thermostat
Dirty Air Filter	Reduced airflow, short cycling	Replace filter every 1–3 months
Flame Or Ignition Problems	Pilot out, repeated ignition attempts	Inspect pilot/igniter, clear debris, call tech if needed
Limit Switch Or Overheating	Furnace runs briefly then shuts off	Ensure airflow, test switch; replace if defective
Blower Motor Or Relay Faults	Fan doesn’t run or runs intermittently	Check power, capacitors, replace motor or relay
Gas Supply Issues	Odors, weak flame, furnace stops	Shut off gas and call utility or HVAC pro

How Furnaces Normally Cycle

A typical gas furnace uses a thermostat signal to start the ignition sequence, establish a steady flame, run the blower to circulate warmed air, then cycle off when the set temperature is reached.

Short cycling occurs when the furnace turns on and off repeatedly before completing a normal heating cycle. Short cycling reduces comfort, raises wear on components, and increases energy use.

Top Causes Why A Furnace Keeps Shutting Off

Thermostat Malfunction Or Settings

Faulty wiring, incorrect programming, dead batteries, or a failing thermostat sensor can cause the furnace to receive intermittent signals, resulting in premature shutoff.

Smart thermostats can experience software glitches or connectivity issues that mimic mechanical faults. Verify settings and perform a basic reset if needed.

Clogged Or Dirty Air Filter

A dirty filter restricts airflow, causing the heat exchanger to overheat. The furnace’s safety limit switch senses excessive temperature and shuts the system down to prevent damage.

Replacing or cleaning the filter is often the simplest fix and should be the first step in most troubleshooting sequences.

Flame Sensor Or Ignition Problems

If the flame sensor is dirty or failing, the control board may interpret the flame as absent and shut off the gas valve. Similarly, a weak spark igniter or inconsistent pilot light can interrupt operation.

Cleaning the flame sensor with a fine abrasive pad or having a technician replace a failing igniter typically resolves these issues.

Limit Switch Or Overheat Cutoff

The limit switch prevents the furnace from overheating by shutting the burners off when temperatures exceed safe thresholds. Frequent tripping points to airflow restrictions, blocked ducts, or a failing switch.

A technician can test the limit switch, diagnose the root cause, and replace the part if defective.

Blower Motor, Capacitor, Or Relay Failures

If the blower motor does not start or stops prematurely, the furnace may turn off because proper airflow is not established. Faulty start capacitors, relays, or the motor itself are common culprits.

Listen for humming, clicking, or no sound from the blower when the furnace cycles on; these signals help pinpoint an electrical component failure.

Gas Supply And Pressure Issues

Insufficient gas pressure, a closed gas valve, or utility interruptions will cause the furnace to initiate ignition but then shut down. A weak flame or repeated ignition attempts often indicate supply problems.

Any suspicion of a gas leak or irregular supply requires immediate safety steps and contacting the gas company or a certified HVAC technician.

Step-By-Step Troubleshooting Guide

Basic Safety Precautions

Before any inspection, turn off the furnace power using the switch or breaker. If a gas odor exists, evacuate occupants, avoid open flames, and call the gas company or emergency services.

Wear protective gloves and glasses when accessing panels or components, and do not attempt gas valve repairs unless qualified and authorized.

1. Check The Thermostat

Confirm thermostat mode is set to “Heat” and temperature setpoint is above room temperature. Replace batteries and verify wiring connections at the thermostat and furnace control board.

To test, temporarily set the thermostat a few degrees higher and observe whether the furnace starts and maintains operation. Swap with a known working thermostat, if available.

2. Replace Or Clean The Air Filter

Locate the filter in the return duct or furnace compartment, inspect for dust and debris, and replace disposable filters or clean reusable ones.

Use filters rated MERV 8–11 for typical homes. After replacement, run the furnace for a normal cycle to see if short cycling stops.

3. Inspect The Flame Sensor And Ignitor

Access the burner assembly and visually inspect the flame. A struggling or absent flame suggests ignition or gas issues. Carefully remove the flame sensor and clean the metal rod with emery cloth or fine sandpaper.

If the furnace uses a hot surface ignitor, check for visible cracks. Replacing a cracked ignitor is common and inexpensive compared to repeated failures.

4. Verify Blower Operation

With the furnace running, listen for the blower startup. If the blower delays or fails, check the fan limit switch, capacitor, and motor connections. A failing capacitor often causes no-start or intermittent running.

Measuring motor voltage and capacitor health requires a multimeter and knowledge of electrical safety; hire a technician if unsure.

5. Check For Error Codes And Diagnostic Lights

Modern furnaces feature LED diagnostic lights and control board error codes. Consult the furnace’s data plate or manual to interpret the code, which can point directly to faulty sensors, pressure switches, or ignition failures.

Document any codes and share them with a technician if professional help is needed to speed diagnosis.

When To Call A Professional HVAC Technician

Contact a licensed HVAC technician when troubleshooting identifies gas leaks, electrical faults, control board failures, persistent limit switch trips, or when the homeowner lacks tools or expertise.

Professionals can perform safety checks, gas pressure testing, control board diagnostics, and component replacement under warranty or code compliance standards.

Estimated Repair Costs And Parts

Costs vary across regions; these are typical U.S. ranges for common repairs: flame sensor replacement $80–$200, ignitor $100–$300, blower motor $300–$900, capacitor $60–$200, thermostats $50–$300, control board $250–$700.

Labor rates vary; always request written estimates and confirm the technician’s license and insurance before authorizing work.

Preventive Maintenance To Reduce Short Cycling

• Replace filters every 1–3 months depending on usage and pets.
• Schedule annual professional tune-ups before the heating season.
• Keep vents and registers unobstructed to maintain airflow.
• Clean around the furnace and ensure proper clearance per manufacturer guidelines.
• Check and calibrate the thermostat periodically and replace old thermostats when unreliable.

Common Misdiagnoses And Myths

Believing the thermostat battery is the cause when the real issue is airflow restriction is a common misdiagnosis. Similarly, repeatedly resetting the furnace without addressing root causes can damage controls.

DIY replacements of gas valves or piping are unsafe and commonly advised against; these tasks are regulated and require certified professionals.

Checklist For A Quick Homeowner Inspection

Task	Action
Power	Confirm furnace switch and breaker are on
Thermostat	Check mode, batteries, and setpoint
Filter	Inspect and replace if dirty
Vents	Ensure supply and return vents are open and unblocked
Ignitor/Flame	Observe ignition behavior and clean flame sensor if accessible
Odors	If gas smell present, shut off gas and call utility

Energy And Safety Considerations

Short cycling increases energy bills and can shorten the life of furnace components. Resolving the issue promptly improves system efficiency and reduces the risk of larger failures.

Carbon monoxide risk increases when combustion is incomplete or venting is compromised. Install and regularly test CO detectors in the home, especially near sleeping areas.

How To Choose The Right HVAC Service

Select a contractor with local licensing, positive reviews, and clear pricing. Ask for maintenance plans, emergency service availability, and manufacturer training on the specific furnace brand.

Get multiple quotes for larger repairs and request warranties for parts and labor. Keep maintenance and repair records to help with resale value and warranty claims.

Resources And Further Reading

Reliable sources include the U.S. Department of Energy for efficiency guidance, the Air Conditioning Contractors of America (ACCA) for service standards, and manufacturer manuals for model-specific troubleshooting.

Homeowners can consult online diagnostic guides from major furnace brands but should avoid unverified repair videos when gas or electrical hazards exist.

Note: This article provides guidance for informational purposes and does not replace professional diagnosis for gas appliances. When in doubt, prioritize safety and contact a licensed HVAC professional.