how to tell if contactor is bad on ac unit


Signs and Symptoms of a Faulty Contactor on an AC Unit

Contactor failure is a common issue in air conditioning units that can cause your system to perform poorly or even cease functioning entirely. Identifying and resolving a faulty contactor promptly is crucial to ensure the optimal performance and longevity of your AC unit. In this article, we will delve into how to tell if a contactor is bad on an AC unit and provide you with actionable steps to diagnose and remedy the issue effectively.

The Basics: What is a Contactor?

Before we delve into the signs of a bad contactor, let's start by understanding what a contactor is and what role it plays in an AC unit. A contactor is an electrical device that controls the flow of electricity to various components of your air conditioning system, including the compressor, fan motor, and condenser coil. It essentially acts as a switch that allows or cuts off the power supply to these components based on signals from the thermostat.

Sign 1: AC Unit Won't Turn On or Off

One of the primary signs of a faulty contactor is an AC unit that won't turn on or off as intended. When a contactor fails, it may get stuck in an open or closed position, resulting in an inability to start or stop your air conditioning system. If you notice that your AC unit doesn't respond to commands from the thermostat or fails to shut off when it reaches the desired temperature, a bad contactor might be the culprit.

If you suspect a faulty contactor, it's important to turn off the power to your AC unit before attempting any diagnostic procedures. Electrical components can be dangerous if mishandled, so always exercise caution and, if needed, consult a professional HVAC technician.

Sign 2: Buzzing or Humming Noises

Another common sign of a failing contactor is the presence of unusual buzzing or humming noises coming from your AC unit. The contactor contains electromagnets that draw power to close or open the circuit. Over time, these electromagnets can wear out or become damaged, causing them to vibrate or hum audibly when activated.

If you hear strange buzzing or humming sounds emanating from your air conditioning system, it is advisable to inspect the contactor. However, remember to turn off the power before attempting any inspection and seek professional assistance if necessary.

Sign 3: AC Unit Runs Intermittently or Cycles Frequently

A malfunctioning contactor can also lead to irregular or frequent cycling of your AC unit. If the contactor is unable to maintain a steady connection, your system may turn on and off in short intervals or run inconsistently. This can result in inadequate cooling, reduced energy efficiency, and potential damage to the compressor.

If you notice unusual and frequent cycling of your air conditioning system, it's important to investigate the contactor as a possible cause. A certified HVAC technician can help diagnose the issue accurately and replace the contactor if needed.

Sign 4: Burnt or Pitted Contacts

Visual inspection is an excellent way to identify a bad contactor. Remove the access panel of your AC unit and locate the contactor, usually found near the compressor. Inspect the contacts for any signs of damage, such as discoloration, pitting, or burning. These issues can occur due to arcing, a phenomenon caused by an electrical discharge between the contacts.

If you notice any visible damage to the contacts, it's crucial to replace the contactor promptly. Continuing to operate your AC unit with damaged contacts can lead to further electrical problems and potential system failure.

Sign 5: Excessive Heat

A failing contactor can generate excess heat due to increased resistance in the electrical circuit. If you notice that the contactor or the surrounding area feels unusually hot to the touch, it may indicate a problem. Overheating can further exacerbate contactor failure and compromise the overall performance and safety of your AC unit.

When dealing with electrical components such as contactors, it's essential to prioritize safety. If you suspect overheating or experience any unusual heat emanating from your air conditioning system, it is advisable to contact a professional HVAC technician. They can assess the situation and undertake any necessary repairs or replacements while ensuring your safety.


In conclusion, a faulty contactor in your AC unit can lead to various problems, from failure to start or stop to reduced energy efficiency and potential component damage. By familiarizing yourself with the signs of a bad contactor, such as an unresponsive system, buzzing noises, frequent cycling, visual damage, or excessive heat, you can take appropriate measures to address the issue promptly.

If you encounter any of these signs, it is advisable to consult a professional HVAC technician. They possess the necessary expertise and tools to diagnose the problem accurately and undertake any required repairs or replacements safely.

Remember, regular maintenance and inspection of your air conditioning system can help prevent contactor failure and other potential issues. By staying vigilant and addressing problems early on, you can ensure the longevity and optimal performance of your AC unit for years to come.


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