how to check the contactor on a lennox ac unit



The contactor in a Lennox AC unit plays a vital role in controlling the flow of electricity to the compressor and the condenser fan motor. Over time, this component may become worn or damaged, leading to issues with your air conditioning system. However, checking the contactor doesn't have to be a daunting task. In this article, we will guide you through the step-by-step process of inspecting the contactor on a Lennox AC unit, allowing you to diagnose and resolve any problems efficiently. By understanding how to check the contactor, you can ensure that your Lennox AC unit operates at peak performance, keeping you cool and comfortable all summer long.

The Role of the Contactor:

The contactor acts as a switch that controls the flow of electricity to the compressor and the condenser fan motor. When the thermostat calls for cooling, a low-voltage signal is sent to the contactor, which energizes and allows high-voltage power to flow to the compressor and the fan motor. Once the desired temperature is reached, the thermostat signal stops, leading the contactor to de-energize and cut off power to the compressor and fan motor.

How to Check the Contactor on a Lennox AC Unit:

To ensure that your Lennox AC unit's contactor is functioning correctly, follow these steps:

Gather the Necessary Tools:

Before starting, gather the tools you will need for this inspection. It includes a multimeter, a screwdriver set, and safety gloves to protect yourself from any potential electrical hazards.

Turn Off the Power:

The first and most crucial step is to turn off the power supply to the air conditioning system. Locate the circuit breaker panel and switch off the breaker that controls the AC unit. Additionally, it is recommended to turn off the disconnect switch, usually located near the outdoor unit, ensuring complete electrical isolation for your safety.

Remove the Access Panel:

Once the power is off, you can start by removing the access panel on the outdoor unit. Use a screwdriver to remove the screws securing the panel. Carefully lift the panel away to reveal the internal components.

Locate the Contactor:

Now that you have access to the internal components, you need to locate the contactor. It is usually a rectangular-shaped component with multiple electrical terminals and a metal coil in the center. The contactor may be covered by a plastic or metal shroud, so you may need to remove it to gain a clear view.

Inspect the Condition of the Contactor:

Once the contactor is visible, closely inspect its condition. Look for signs of wear, such as burning or charring on the contacts, which could indicate arcing or excessive heat buildup. Excessive vibration or loose connections may also be evident, requiring further investigation. Additionally, check for any signs of corrosion or rust, as this can affect the contactor's performance.

Check for Voltage:

To ensure that your contactor is receiving voltage, use a multimeter to measure the electrical current passing through it. Set the multimeter to the appropriate voltage range and carefully touch the contactor's terminals with the meter's probes. If no voltage is detected, it may indicate a problem with the control board or the low-voltage wiring leading to the contactor.

Test the Contactor's Coil:

To test the coil of the contactor, set the multimeter to measure resistance (ohms). Carefully touch the probes to the terminals of the coil. A properly functioning coil should show a resistance reading within the manufacturer's specifications. If the reading is outside the acceptable range, it may indicate a faulty coil that needs to be replaced.

Inspect the Contacts:

The contacts within the contactor are crucial for allowing the flow of electricity. Look for signs of pitting, corrosion, or excessive wear on the contacts. Additionally, check for any buildup of dirt or debris that may hamper proper contact. If the contacts appear damaged or dirty, it is recommended to clean or replace the contactor.

Test the Contactor's Operation:

To test the contactor's operation, you will need an assistant. When the power is turned back on at the circuit breaker panel, have your assistant lower the thermostat to make a cooling call. Observe the contactor closely for any signs of movement. You should hear a distinct clicking sound as the contactor energizes and allows power to flow. If there is no movement or clicking sound, it may indicate a faulty contactor that needs replacement.

Replace the Contactor:

If, after careful inspection and testing, you determine that the contactor is faulty, it is best to replace it with a new one. Contact your local Lennox dealer or an HVAC professional to obtain the correct replacement part. Follow the manufacturer's instructions to ensure proper installation and functionality.


The contactor on your Lennox AC unit is an essential component responsible for controlling the flow of electricity to the compressor and the condenser fan motor. By knowing how to check and maintain the contactor, you can ensure optimal performance and prolong the lifespan of your air conditioning system. Remember always to turn off the power before inspecting or replacing any electrical components, and if you are unsure, it is best to consult a professional HVAC technician. Regular maintenance and care will help keep your Lennox AC unit running smoothly, providing reliable cooling for years to come.


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