How to Troubleshoot a Lennox Heater Model C33-60D-2F


The Lennox C33-60D-2F is part of the C33 line of commercial-grade split systems with indoor coils. The C33 split system has an upflow configuration with a large coil area. The coils are made with aluminum fins and the system tubing is copper-rifled. In addition, the cabinet and drain pans are all painted and galvanized to help protect against wear and corrosion. If problems occur with the Lennox system, the error light on the control panel will flash to indicate an issue that requires troubleshooting.

Things You'll Need

  • Steel wool pad
  • Connect the Lennox C33 system to an electrical outlet with at least 120 volts of power if the unit is not running and none of the control panel lights are illuminated or the unit will not run continuously even when set to “Auto” or “Fan.” Replace the circuit breaker if needed.

  • Open and close the main access control door firmly to engage the door switch if the unit has power but the control panel lights are not illuminated. Contact Lennox for a replacement control board if the problem continues.

  • Unplug the Lennox unit and plug back into a working power outlet if the first LED light flashes quickly and the second light flashes slowly, indicating that the air inducer is not energized or reverse polarity has occurred. Plug the unit into a grounded outlet with more than 75 volts if both LED lights flash quickly, indicating that the Lennox unit requires proper grounding and voltage.

  • Press the top button on the control panel if the first LED light stays on and the second light flashes slowly, indicating a roll-out error. Allow the unit to automatically reset. Contact Lennox if the error lights continue after pressing the reset button and powering up the unit, as the pin connector may have a low voltage connection to the control board.

  • Shut down the power on the Lennox unit if the first LED light flashes slowly and the second LED light stays lit, indicating that the primary or secondary limit is open. Let the unit cool down, which automatically resets each switch. Check the inlet and outlet air vents and registers and clear any blockages before restarting the unit. Look at the inlet and outlet vents if the first LED light stays off and the second LED light flashes slowly, which may indicate a low differential pressure on the pressure switch caused by an obstruction at one of the vents.

  • Turn open the manual gas valve if both LED lights flash slowly, indicating that there is low gas supply. Look at the gas valve gauge, which should not read more than 13 inches water column (WC). Turn off the furnace and evacuate the building if there is the scent of gas and then call the gas company or emergency services.

  • Disconnect power to the Lennox unit and allow the unit to cool and then open the main access control panel and use a steel wool cleaning pad to carefully clean the sensing rod on the burner. Reconnect the power.

Tips & Warnings

  • Contact Lennox or another qualified service professional if both lights flash slowly, indicating a failed combustion air inducer motor, loose wiring, sensor errors, an open ignitor circuit or a failed ignitor. If the first LED light does not illuminate and the second LED light flashes slowly, the furnace or combustion air inducer may be wired incorrectly or the pressure switch may be stuck closed and needs to be replaced. Alternating slow flashes on both lights indicate that the gas valve may not have enough voltage or a damaged vent or burner. These issues require professional service. You should also contact a qualified service professional if troubleshooting efforts do not resolve the problem.

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