how to troubleshoot ac contactor


What is an AC Contactor and Why is Troubleshooting Important?

AC contactors are an essential component in air conditioning systems, responsible for controlling the flow of electricity to the unit's compressor. These contactors ensure that the compressor starts and stops according to the system's demands, allowing for efficient cooling and temperature regulation. However, like any other electrical component, AC contactors can sometimes encounter problems that require troubleshooting.

Troubleshooting AC contactors is crucial for maintaining the performance and functionality of your air conditioning system. By identifying and rectifying issues promptly, you can prevent potential breakdowns, costly repairs, and even extend the lifespan of your system. In this article, we will explore common issues that can arise with AC contactors and guide you through the troubleshooting process to help you resolve these problems effectively.

Common Issues with AC Contactors

AC contactors may encounter several issues over time due to various factors, such as wear and tear, electrical fluctuations, or dirt accumulation. Understanding these common issues will enable you to identify the root cause of the problem and proceed with troubleshooting accordingly.

1. Failure to Start

One of the most prevalent problems with AC contactors is the failure to start the compressor. When this occurs, it typically indicates an issue with the contactor's coil or contacts. The coil may have burned out or become damaged, preventing it from creating the electromagnetic force required to close the contacts. In some cases, the contacts themselves might have become worn or fused together, hindering their ability to connect the electrical circuit.

To troubleshoot this problem, it is essential to start by examining the contactor visually. Look for any signs of damage or burnt components. If the coil appears damaged, it may need to be replaced. Additionally, check the contacts for signs of wear or fusion. If the contacts are worn, they can be replaced. However, fused contacts may require replacing the entire contactor unit.

2. Failure to Stop

In contrast to the previous issue, an AC contactor failing to stop can be equally troublesome. This problem can occur due to a faulty coil or contacts that have become fused together, preventing them from opening the electrical circuit when the system reaches the desired temperature. The contactor might also fail to stop if it is not receiving the signal to disconnect from the thermostat.

Troubleshooting this issue involves inspecting the coil and contacts for any damage or signs of fusion. If the coil is damaged or burned out, replacing it should resolve the problem. On the other hand, fused contacts necessitate replacing the entire contactor. If the contactor seems fine, check the wiring connections and the thermostat for any faults that may be causing the issue.

3. Buzzing or Humming Sounds

Buzzing or humming sounds coming from the contactor can be an indication of loose components, inadequate power supply, or impending failure. Loose components, such as screws or wires, can vibrate when electricity flows through the contactor, resulting in these irritating sounds. Insufficient power supply to the contactor coil can also cause buzzing or humming.

Start troubleshooting this issue by ensuring all components are securely fastened. Tighten any loose screws or wires attached to the contactor. If the buzzing persists, check the power supply voltage to the contactor coil using a multimeter. Compare the measured voltage to the manufacturer's specifications. If the voltage is significantly lower than recommended, you may need to consult an electrician to address the power supply issue.

4. Contact Sticking

Contact sticking refers to the contacts in the contactor failing to open or close fully. This problem can occur due to contamination by dirt, dust, or corrosion, hindering the smooth movement of the contacts. Contact sticking can cause electrical arcing and increased friction, potentially leading to contactor failure or even a system malfunction.

To troubleshoot contact sticking, turn off the power supply to the air conditioning system and carefully inspect the contactor. If you notice any visible dirt or corrosion, clean the contacts using electrical contact cleaner and a soft, non-metallic brush. Gently remove any debris without damaging the contacts or surrounding components. Once cleaned, retest the operation of the contactor to determine if the issue has been resolved.

5. Excessive Noise or Vibration

If your AC contactor is generating excessive noise or causing vibrations in the air conditioning system, it can detract from the overall comfort and efficiency of your unit. Excessive noise and vibration can be caused by loose mounting, misalignment, or a failing contactor.

Start troubleshooting this issue by inspecting the mounting of the contactor. Ensure that it is securely attached and that all screws or bolts are tightened. You may also want to check for any signs of misalignment between the contactor and other components. If the mounting and alignment are fine, but excessive noise and vibration persist, it may be an indication of a failing contactor. In this case, replacing the contactor should resolve the problem.


Troubleshooting AC contactor issues is essential for maintaining the smooth operation of your air conditioning system. By identifying and resolving problems promptly, you can prevent disruptions, costly repairs, and potential damage to other components. Remember to check for common issues like failure to start or stop, buzzing or humming sounds, contact sticking, and excessive noise or vibration. Performing troubleshooting steps such as visual inspections, cleaning, and component replacements when necessary can help sustain the performance and extend the lifespan of your AC contactor. If you encounter difficulties or are uncertain about any step, it’s advisable to consult a professional HVAC technician to ensure accurate diagnosis and resolution of the problem.


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