Furnace Blower Starts Then Stops: Causes, Troubleshooting and Fixes

The furnace blower starting then stopping is a common HVAC issue that interrupts heating, increases energy costs, and can signal safety concerns. This article explains likely causes, diagnostic steps technicians and homeowners can follow, and practical fixes to restore reliable blower operation.

Symptom	Possible Causes	Priority
Blower runs briefly then stops	Overheat/limit switch trip, clogged filter, motor overheating	High
Blower cycles with short pauses	Faulty relay/contactor, ECM control issue	Medium
Blower runs intermittently	Thermostat wiring, control board faults	Medium
Blower fails to restart after pause	Capacitor failing, motor winding issues	High

How A Furnace Blower System Works

The furnace blower moves warmed air through ductwork using an electric motor and fan wheel. A typical system includes a thermostat call, control board or relay, motor (PSC or ECM), blower wheel, air filter, and safety switches such as the limit switch and rollout switches. Understanding this chain helps isolate causes when the blower starts then stops.

Common Causes When The Blower Starts Then Stops

Overheating And Limit Switch Trips

A clogged filter, blocked return vents, closed dampers, or restricted ductwork can make the furnace overheat. When internal temperature exceeds safe limits, the limit switch opens and cuts power to the blower or burner to prevent damage.

Faulty Blower Motor Or Capacitor

Single-phase motors use a start/run capacitor. A failing capacitor or aging motor may allow the blower to start briefly before voltage drops and the motor stalls or trips thermal protection. Symptoms include sputtering start and no consistent speed.

ECM/Control Board Malfunctions

Modern furnaces often use electronically commutated motors (ECMs) with digital controls. Software or hardware faults on the control board can cause intermittent signals, making the blower cycle or stop unexpectedly. Diagnostics codes on the board or LED blink patterns are useful clues.

Blower Relay Or Contactor Issues

The relay (electromechanical switch) that supplies the blower can fail or have burnt contacts. When the relay welds or loses contact integrity, it may click, supply power momentarily, then fail, causing the blower to stop.

Thermostat Or Wiring Problems

Faulty thermostat signals, shorted wiring between the thermostat and furnace, or loose connectors can cause intermittent blower operation. Check for consistent 24V control signals during a heating call.

Flame Sensor, Pressure Switch, Or Safety Switch Interference

While primarily involved in burner operation, a tripped pressure switch or flame sensor fault can shut down the burner and influence fan control sequences, making the blower change states unexpectedly.

Diagnostic Checklist For Homeowners And Technicians

Start with safe, noninvasive checks and progress to electrical diagnostics. Always turn off power before handling internal components.

1. Inspect Air Filter And Vents: Replace dirty filters and ensure supply and return vents are open and unobstructed.
2. Check For Error Codes: Observe the furnace control board LED or display for diagnostic blink codes and note patterns for technician reference.
3. Listen For Relay Clicks: When the blower stops, listen for relay or contactor noise. A relay that clicks repeatedly may be failing.
4. Measure Voltage: Using a multimeter, confirm 120/240V to the motor and 24V control signals at the relay during a call for heat (for qualified DIYers/technicians).
5. Examine Capacitor: Look for bulging or leaking capacitors. Measure capacitance if possible; a failing capacitor often causes starting then stopping.
6. Check Motor Temperature: Warm or hot-to-touch motors may have tripped internal thermal protection; allow cool-down and retest.
7. Inspect Ductwork: Restrictive ducts or closed dampers can produce high static pressure, straining the blower motor.

Step-By-Step Troubleshooting Procedures

1. Address Airflow Restrictions First

Replace the air filter and clear returns. If airflow is restored and the blower runs consistently, airflow restriction was likely the trigger for limit trips.

2. Verify Thermostat And Control Signals

Set thermostat to call for heat and verify the furnace control receives the 24V signal. If the signal is intermittent, inspect thermostat wiring at both ends and swap in a known good thermostat if available.

3. Test The Capacitor And Motor

With power off, discharge and test the capacitor with a meter. Replace if beyond tolerance. If the motor hums or stalls without the fan blade spinning smoothly, motor bearings or windings may be failing.

4. Monitor Limit Switch Behavior

Observe whether the limit switch opens during operation. If it does, measure return air temperature and airflow. Persistent trips indicate an airflow or combustion problem that requires correction.

5. Inspect The Control Board And Relay

Look for burned traces, swollen components, or loose connectors. If the relay is worn or contacts are pitted, replace the relay or board as needed. For ECM-driven systems, check for firmware error codes before replacing the motor.

When To Replace Parts Versus Repair

Decisions depend on age, cost, efficiency, and safety. Replace capacitors and relays early as inexpensive preventive fixes. Motors older than 10–15 years with winding or bearing failure often merit replacement. Control board replacement is appropriate when diagnostics point to failed electronics.

DIY Fixes Homeowners Can Safely Perform

• Replace dirty air filters and clear vents.
• Tighten loose thermostat wiring at the thermostat itself.
• Reset power at the furnace to clear temporary faults and observe behavior.
• Clean blower wheel dust buildup using a brush if easily accessible and power is disconnected.

More advanced electrical checks should be left to qualified technicians due to shock risk and potential furnace damage.

When To Call A Professional

Contact a licensed HVAC technician if the blower continues to stop after basic checks, if there are signs of electrical burning, unusual smells, loud motor noise, or repeated safety trips. Professionals can safely test capacitors, motors, control boards, and combustion systems and provide permanent repairs.

Preventive Maintenance To Avoid Future Start-Stop Problems

Annual HVAC tune-ups reduce the chance of blower cycling issues. Recommended steps include replacing filters every 1–3 months, cleaning the blower assembly and ducts, lubricating motor bearings where applicable, and verifying control board and thermostat operation.

Costs And Typical Repair Timeframes

Costs vary by region and system type. Filter replacement is low-cost. Capacitor replacement typically runs $100–$250 including labor. Motor replacement can range $400–$1,200 depending on furnace model and labor. Control board replacement may cost $300–$800. Most repairs are completed within 1–3 hours, while major motor swaps can take longer.

Safety Considerations And Building Codes

Do not bypass limit switches or safety interlocks to force the blower to run. Tampering with safety devices can create fire or carbon monoxide risks and violate codes. Electrical repairs should comply with local code and be performed or inspected by licensed professionals when required.

Troubleshooting Reference Table

Symptom	Quick Check	Likely Fix
Blower runs briefly then stops	Replace filter, inspect limit switch	Clear airflow restrictions, replace limit or motor if failing
Blower starts humming, then stops	Inspect capacitor	Replace capacitor or motor
Blower cycles every few minutes	Listen for relay clicks, check control board LEDs	Replace relay/control board
Blower runs intermittently with no airflow change	Test thermostat signal	Repair wiring or thermostat

Key Takeaways For Homeowners

1) Start with airflow and filter checks because they are common, low-cost causes. 2) Capacitors and relays are frequent culprits when the blower starts then stops. 3) Repeated safety trips require immediate professional attention to avoid hazards.

Accurate diagnosis saves time and cost: document symptoms, listen for patterns, and share control board codes with the technician to speed repairs.

Useful Resources And Further Reading

• Energy Star: Home Heating Tips
• Air Conditioning Contractors Of America (ACCA)
• ASHRAE: HVAC Resources