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The Tech What

AC compressor not turning on

Troubleshooting Guide: AC Compressor Not Turning On


When your AC compressor fails to start, it can lead to a lack of cooling and discomfort, especially during hot weather. This guide provides a systematic approach to diagnose and resolve the issue of an AC compressor not turning on.

Understanding the Problem

Causes of AC Compressor Not Turning On

Several factors can contribute to the AC compressor not starting, affecting the overall functionality of your air conditioning system.

Common Causes and Solutions

1. Electrical Issues

a. Tripped Circuit Breaker

Check the circuit breaker panel to ensure the breaker for your AC unit hasn’t tripped. A tripped breaker can cut off power to the compressor, preventing it from starting. Reset the breaker and monitor if the compressor starts.

b. Faulty Capacitor

The capacitor provides the initial electrical boost needed to start the compressor. If the capacitor is defective or worn out, it may not be able to provide sufficient power to start the compressor. Inspect the capacitor for signs of bulging or leaking and replace if necessary.

2. Thermostat Problems

a. Incorrect Settings

Ensure that the thermostat is set to “Cool” and the temperature is set lower than the current room temperature. Incorrect thermostat settings can prevent the compressor from activating.

b. Malfunctioning Thermostat

If the thermostat is malfunctioning, it may not send the signal to start the compressor. Test the thermostat by setting it to a lower temperature and listening for a click indicating that it’s sending a signal.

3. Compressor Issues

a. Overheating

An overheated compressor may have a safety mechanism that prevents it from starting until it cools down. Check for any signs of overheating such as a hot exterior or a burning smell.

b. Mechanical Failure

If the compressor motor or its internal components have failed, it will need to be replaced. This is often indicated by clicking sounds or humming without the compressor starting.

Steps to Troubleshoot and Resolve

Step 1: Check Power Supply

Verify that the AC unit is receiving power by checking the circuit breaker. Reset any tripped breakers and test the unit to see if the compressor starts.

Step 2: Inspect and Replace Capacitor

Examine the capacitor for visible signs of damage or wear. Replace the capacitor if it appears faulty to ensure adequate power supply for the compressor.

Step 3: Test Thermostat Operation

Set the thermostat to “Cool” and lower the temperature setting. Listen for a click to confirm that the thermostat is sending the signal to start the compressor.

Step 4: Evaluate Compressor Condition

Inspect the compressor for any visible signs of overheating or mechanical failure. Contact a qualified HVAC technician to diagnose and replace the compressor if necessary.


Dealing with an AC compressor that isn’t turning on requires systematic troubleshooting to identify and resolve the underlying issues. By following the steps outlined in this guide, homeowners can often diagnose and fix the problem themselves. If troubleshooting proves challenging or if components need replacement, it’s advisable to seek assistance from a professional HVAC technician to ensure your system functions efficiently.