Lennox Furnace Blower Won’t Shut Off — What’s Happening
When your Lennox furnace blower keeps running after the heating cycle ends or even when the thermostat is off, the system is usually protecting itself from overheating. The control board forces the blower to run continuously when the high-limit switch trips due to restricted airflow or other fault conditions. This safety feature prevents the heat exchanger from warping or cracking by pushing air across it until internal temperatures drop below 90 to 95 degrees.
Less often, the blower runs nonstop because of a mechanical failure like a stuck relay or a wiring short that sends a constant 24-volt signal to the blower circuit. On communicating Lennox systems like iQR or XL series, a motor driver board failure can also lock the blower in the on position. The furnace may display codes like E201 for blower communication failure or a limit switch fault, but sometimes no code appears at all.
Most Likely Causes
- Dirty air filter or blocked return Restricted airflow causes the heat exchanger to overheat, trips the high-limit switch, and the board runs the blower continuously to cool the unit.
- Thermostat fan setting on ON instead of AUTO The thermostat is manually commanding the blower to run nonstop regardless of heating demand.
- Stuck sequencer relay or blower relay Relay contacts weld closed from arcing or age, keeping the blower circuit energized even when the control board cuts power.
- Shorted thermostat wiring (G to C or G to R) A wire short between the fan wire and common or power wire sends constant voltage to the blower relay.
- Faulty flame sensor causing repeated cycling Dirty or failing flame sensor prevents ignition confirmation, and the board runs the blower to purge unburned gas after failed attempts.
- Over-firing from incorrect gas pressure Manifold pressure set too high causes rapid overheating and triggers limit protection mode with continuous blower operation.
- Communicating blower motor driver failure (E201) Internal motor control board shorts or loses communication with the main board, defaulting the motor to constant run.
How to Diagnose and Fix
- Check the thermostat fan switch and confirm it is set to AUTO, not ON.
- Turn off power at the furnace disconnect and inspect the air filter, replacing it if dirty or clogged.
- Restore power and observe the blower behavior, then check the furnace display for fault codes like E201 or limit switch errors.
- Turn off power again and remove the thermostat from the wall plate, then separate the G wire from the terminal and restore power to see if the blower stops.
- With power off, open the blower compartment and inspect the blower relay on the control board for visible welding or burn marks on the contacts.
- If the relay looks damaged, turn power on briefly and listen for a click when you turn the thermostat to OFF, then measure 24 volts AC at the relay coil terminals to confirm the board is de-energizing the relay.
- Check all ductwork for crushed sections or closed dampers that restrict airflow, and open at least half the supply registers in the home.
- If codes indicate flame or gas valve issues or you suspect over-firing, call a licensed technician to measure manifold gas pressure and inspect the burner assembly and flame sensor.
Parts You Might Need
| Part | Notes |
|---|---|
| Blower relay or sequencer | Amazon | Match the coil voltage (24V typical) and contact rating to your Lennox model control board |
| Furnace air filter | Amazon | Check size on existing filter frame, replace with correct MERV rating per manufacturer specs |
| High-limit switch | Amazon | Rated for your heat exchanger type, resets at 90 to 95 degrees, verify part number from unit label |
Related Error Codes
If your appliance also shows a code on the display, these match this problem:
- Lennox Furnace E117 error code
- Lennox Furnace E125 error code
- Lennox Furnace E204 error code
- Lennox Furnace E205 error code
- Lennox Furnace E206 error code
- Lennox Furnace E227 error code
- Lennox Furnace E241 error code
- Lennox Furnace E250 error code
- Lennox Furnace E270 error code
- Lennox Furnace E275 error code
- Lennox Furnace E290 error code
When to Call a Pro
Call a licensed HVAC technician if the furnace displays flame failure codes, if you smell gas, or if the blower continues running after you have replaced the filter and confirmed the thermostat is in AUTO mode. Gas pressure testing, flame sensor cleaning, and heat exchanger inspection require calibrated tools and safety protocols. Any work involving the gas valve, burner assembly, or internal wiring on communicating Lennox boards should be handled by a professional to avoid voiding warranties or creating fire hazards. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.