Furnace locks out with LED code 13 (limit circuit lockout) after repeated limit trips; blower may run continuously.

Safety: Turn off power and gas before working inside the furnace cabinet.

Steps:

• Verify filter and coil cleanliness: Replace the furnace filter. If accessible, inspect the evaporator coil above the furnace for dust or ice buildup. A heavily plugged coil must be cleaned by a technician.
• Check blower wheel: Remove the blower assembly and inspect the blower wheel for dust buildup on the blades. Clean with a soft brush and vacuum, taking care not to bend the fins.
• Confirm blower speed tap: On PSC motor models, verify that the heating speed tap is connected to the correct terminal on the control board as per the Keeprite wiring diagram. Incorrect low speed can cause overheating.
• Inspect duct sizing and restrictions: Look for undersized or long duct runs, closed dampers, or restrictive aftermarket filters. Open all dampers and registers fully.
• Reset lockout: After correcting issues, restore power to clear the lockout. Call for heat and monitor operation through a full cycle.

Call a pro if: The furnace continues to trip the limit or you suspect improper sizing or a defective limit switch; these require professional testing and possible replacement.