how to test if an ac contactor is bad



Have you noticed that your air conditioner is not working properly? Is it failing to cool your home or making strange buzzing noises? If so, it's possible that the AC contactor is the culprit. The AC contactor is an important component of your air conditioning system that controls the flow of electricity to the compressor. When it goes bad, your AC unit may not function as it should. In this article, we will discuss how to test if an AC contactor is bad and what signs to look out for. By understanding the symptoms of a faulty contactor, you can take appropriate action and get your air conditioner up and running efficiently.

Signs of a Bad AC Contactor:

A malfunctioning AC contactor can manifest itself through various signs and symptoms. While it's important to note that these signs can also indicate other issues with your air conditioning system, a faulty contactor often lies at the root of the problem. Here are some common signs that may indicate a bad AC contactor:

1. AC Unit Fails to Start:

If you are experiencing difficulty in starting your AC unit, a malfunctioning contactor could be the culprit. The contactor's job is to send the electrical current to the compressor, initiating the cooling process. When the contactor fails, it can prevent the compressor from receiving the necessary power supply, causing your AC unit to fail to start. This is a clear indication that the contactor needs to be tested and possibly replaced.

2. AC Unit Won't Turn Off:

On the flip side, a faulty AC contactor can also result in your air conditioner failing to turn off. When the contactor gets stuck in its closed position, it continues to supply electricity to the compressor, even when the thermostat signals the system to shut down. This can lead to constant running of the AC unit, which not only wastes energy but also puts strain on the components of the system. If your AC unit won't turn off, it's crucial to assess the condition of the contactor.

3. Strange Noises:

Unusual buzzing or humming sounds coming from your air conditioner can be an indication of a bad contactor. When the contactor becomes faulty, it may not close or connect properly, resulting in arcing or sparking. This can generate a buzzing sound, which is often a sign of an electrical issue. If you hear strange noises coming from your AC unit, it's advisable to investigate the contactor as a potential cause.

4. Burn Marks or Melting:

A visual inspection of the AC contactor can reveal important clues about its condition. If you notice burn marks or any signs of melting on the contactor, it is a clear indication of a problem. Burn marks occur when the electrical connections inside the contactor become loose or corroded, leading to heat buildup. Over time, this heat can cause damage to the contactor, compromising its functionality.

5. AC Unit Runs Intermittently:

Another symptom of a bad AC contactor is when your air conditioner runs intermittently. The contactor may be working inconsistently, causing the AC unit to turn on and off at irregular intervals. This intermittent behavior disrupts the cooling cycle and can impact the comfort of your home. If you observe your AC unit behaving erratically, it's time to examine the contactor as a possible culprit.

How to Test an AC Contactor:

Now that we have discussed the signs of a bad AC contactor, let's delve into the process of testing it. Before proceeding with any testing, it's important to ensure your safety by disconnecting the power supply to your AC unit. Follow these steps to test the functionality of your AC contactor:

1. Turn Off the Power:

The first step is to locate the electrical disconnect box for your air conditioner and turn off the power. This is crucial to avoid any electrical shocks during the testing process. The disconnect box is typically located near the outdoor unit or the main electrical panel.

2. Remove the Access Panel:

Next, remove the access panel from your AC unit to gain access to the contactor. The access panel is usually secured with screws or bolts and can be found on the side or top of the unit. Use the appropriate tool to carefully remove the panel, ensuring not to damage any components in the process.

3. Inspect the Contactor:

Once the access panel is removed, visually inspect the contactor for any signs of damage such as burn marks, melting, or loose connections. Pay close attention to the electrical wires and terminals for any signs of wear or corrosion. If you notice any significant damage, it's best to replace the contactor.

4. Test the Continuity:

To test the continuity of the contactor, you will need a multimeter. Set the multimeter to the ohms or continuity setting. Start by testing the control coil, which is the part responsible for closing and opening the contacts. Place one probe on one terminal of the coil and the other probe on the opposite terminal. If the multimeter shows a reading of zero or close to zero ohms, the coil is in good condition. If there is no continuity, it indicates a faulty contactor.

5. Check the Contacts:

Next, test the contacts of the contactor to ensure they are functioning properly. With the multimeter set to the ohms or continuity setting, place one probe on one of the main power terminals and the other probe on the opposite terminal. If the multimeter shows a reading of zero or close to zero ohms, it means the contacts are closed and conducting electricity correctly. On the other hand, if there is no continuity, it indicates that the contacts are either stuck or worn out, requiring replacement.


In conclusion, a bad AC contactor can significantly impact the performance of your air conditioning system. By familiarizing yourself with the signs of a faulty contactor and testing it using the steps outlined in this article, you can diagnose the issue and take the necessary steps to resolve it. Remember, if you are uncertain or uncomfortable performing the testing yourself, it's always best to seek the assistance of a professional HVAC technician. Regular maintenance and prompt repairs will ensure the longevity and efficiency of your air conditioning system, providing you with a comfortable environment all year round.


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