Furnace Inducer Motor Starts and Stops Intermittently

The furnace inducer motor plays a critical role in safe furnace operation by exhausting combustion gases and enabling ignition; when the inducer motor starts and stops intermittently, it can lead to furnace lockouts, inefficient heating, and potential safety concerns. This article explains common causes, step-by-step troubleshooting, diagnostic checks, repair options, cost estimates, and preventive maintenance to help homeowners and technicians identify and resolve inducer motor cycling problems.

Problem	Common Causes	Likely Fix
Inducer Motor Starts Then Stops	Blocked vent, pressure switch fault, wiring, control board	Clear vent, replace switch, repair wiring, update board
Motor Runs Briefly Then Shuts Off	Failing motor, capacitor issue, overheating	Motor or capacitor replacement
Intermittent Cycling With Lockouts	Limit switch trips, flame rollout, condensate switch	Inspect heat exchanger, replace switches, clear drain

How The Inducer Motor Works

The inducer motor powers a small fan that pulls combustion byproducts through the heat exchanger and pushes them out the vent pipe. It runs during the furnace start sequence before ignition and often runs during operation to support draft. Proper inducer function ensures safe combustion and prevents dangerous exhaust gases like carbon monoxide from entering the home.

Common Reasons The Inducer Motor Starts And Stops

1. Venting Obstructions And Pressure Problems

Blocked or restricted venting prevents proper draft, causing the pressure switch to open and stop the inducer. Debris, bird nests, ice, or collapsed vent pipes are common offenders. Weak draft can also be caused by negative house pressure from exhaust fans or tight sealing.

2. Faulty Pressure Switch

The pressure switch senses draft created by the inducer motor. If it is sticking, poorly calibrated, or the tubing is clogged or disconnected, the furnace will detect a failure to prove venting and shut the inducer off. Pressure switch failures are among the most common causes of intermittent inducer cycling.

3. Motor Or Capacitor Failure

Inducer motors can develop worn bearings, electrical winding issues, or bad capacitors. A motor that struggles to start may briefly run and then overheat or trip an internal thermal cutoff, causing it to stop until it cools down.

4. Control Board Or Relay Faults

Modern furnaces rely on a control board to sequence inducer operation. Defective relays, solder joints, or control logic can intermittently cut power to the inducer motor. Control problems often produce inconsistent or unpredictable cycling.

5. High Limit Or Overheat Conditions

If the furnace overheats due to restricted airflow at the blower, clogged filters, or closed registers, the high limit switch may open and stop the inducer as part of a safety shutdown. The inducer may then restart after the furnace cools and the limit resets.

6. Condensate Or Flame Rollout Switches

On high-efficiency condensing furnaces, a blocked condensate drain or clogged trap can trigger a safety switch that stops the inducer. Flame rollout switches triggered by heat exchanger issues will also interrupt inducer operation to protect the home.

7. Thermostat Or Wiring Issues

Defective thermostat signals, loose wire connections, or corroded terminals can cause the control system to intermittently call for heat or drop the call, resulting in inducer start/stop behavior. Electrical faults between components produce similar symptoms.

Safety Precautions Before Troubleshooting

Always follow safety steps before inspecting a furnace. Turn power off at the service switch and the breaker. Allow components to cool. If the furnace is gas-fired, shut off the gas if a leak or strong smell of gas is suspected, and ventilate the area. When in doubt, consult a licensed HVAC technician.

Step-By-Step Troubleshooting Guide

Initial Visual And Sound Checks

Listen to the inducer during startup: a smooth spin indicates mechanical health; grinding, squealing, or repeated clicking suggests motor or bearing issues. Inspect the vent termination for obstructions and look for obvious wiring damage or disconnected hoses.

Check The Air Filter And Blower

A severely clogged filter or blocked return can cause overheating and safety shutdowns. Replace the filter, verify blower operation, and ensure supply registers are not closed. Proper airflow reduces limit switch trips and stabilizes inducer operation.

Inspect The Venting And Pressure Tubing

Clear debris from outdoor vent caps and remove any kinks or blockages in the pressure switch tubing. Use a vacuum or compressed air to clear the pressure hose, and check for moisture or condensation that could be affecting the switch.

Test The Pressure Switch

With the furnace powered and in call-for-heat, measure across the pressure switch terminals using a multimeter; it should close when the inducer starts. If it fails to close or is inconsistent, replace the pressure switch or correct tubing/venting problems first.

Measure Inducer Motor Voltage And Current

Measure supply voltage at the motor terminals to confirm consistent power during operation. Also check motor current draw; abnormally high amperage indicates mechanical binding or failing bearings, while low or fluctuating amperage suggests electrical problems or a failing capacitor.

Assess The Capacitor

If the motor uses a start/run capacitor, test it with a capacitance meter. A failing capacitor often causes slow starting, humming, or short-run behavior. Replacing capacitors is inexpensive and commonly resolves starting issues.

Examine Control Board And Relay Operation

Observe whether the control board consistently sends power to the inducer when it should. Inspect the board for burnt components or cold solder joints. If relays chatter or lose actuation intermittently, the board may need repair or replacement.

Look For Error Codes And Diagnostic Lights

Modern furnaces provide LED flash codes for fault conditions. Consult the furnace manual to decode the blinking pattern, which can point directly to pressure switch problems, inducer faults, limit trips, or control board errors.

When To Repair Vs Replace The Inducer Motor

If the inducer motor shows mechanical noise, high current draw, or intermittent operation despite correct voltage and a good capacitor, replacement is usually the best long-term solution. Motors older than 10–15 years often justify replacement due to reliability and efficiency gains.

Estimated Costs And Timeframes

Repair Item	Typical Cost Range (US)	Time
Pressure Switch Replacement	$75–$250	30–90 minutes
Inducer Motor Replacement	$300–$800	1–3 hours
Control Board Replacement	$200–$600	1–2 hours
Capacitor Replacement	$25–$120	15–45 minutes

Costs vary by furnace model and labor rates. Complex diagnostic work or encountering additional problems like heat exchanger damage will increase the total expense and time required.

Preventive Maintenance To Avoid Inducer Cycling Problems

• Change air filters every 1–3 months depending on usage and filter rating.
• Schedule annual HVAC inspections and tune-ups to catch failing parts early.
• Keep vent terminations clear of debris, snow, and nests.
• Maintain proper airflow in the home—avoid constant negative pressure from exhaust fans.
• Flush and clear condensate drains on high-efficiency furnaces seasonally.

DIY Fixes Homeowners Can Safely Try

Homeowners can clean outdoor vent caps, replace air filters, clear pressure tubing, and visually inspect for obvious wiring issues. Using a multimeter for basic voltage checks is acceptable for those comfortable with electrical tools, but major repairs like inducer motor or control board replacement are best left to professionals.

When To Call A Professional

Call a licensed HVAC technician if the inducer motor continues to cycle after basic checks, if diagnostic codes indicate pressure switch or control board faults, or when gas components and heat exchanger integrity are suspected. A professional can perform safe combustion analysis, carbon monoxide testing, and certified repairs.

Parts And Replacement Tips

When replacing an inducer motor or pressure switch, use OEM parts specified for the furnace model to ensure proper fit, safety, and warranty compliance. If replacing the control board, note all wire locations, take photos before disconnection, and verify firmware or configuration settings if applicable.

Diagnostic Checklist For Technicians

Check	Expected Result
Vent Termination	Clear, unobstructed
Pressure Tubing	No kinks, dry, connected
Pressure Switch	Closes on inducer start
Inducer Motor Voltage	Nominal supply voltage present
Motor Current	Within manufacturer spec
Capacitor	Within tolerance
Control Board	No burnt components, stable relays

Key Troubleshooting Summary

Start By Checking airflow and venting, then inspect the pressure switch and tubing. Test for correct voltage and motor current, evaluate the capacitor, and review control board diagnostics. Address simple issues first—clear vents and replace filters—before replacing major components.

Useful Resources And References

• Energy Star — Furnace Efficiency And Maintenance Tips
• ACCA — HVAC Professional Guidance And Best Practices
• NFPA — Safety Codes For Fuel-Burning Appliances

For persistent starting and stopping of the furnace inducer motor, combining careful troubleshooting with timely professional assistance will restore reliable operation and maintain home safety.