If your AC is not working, the issue could be as simple as a tripped circuit breaker or as serious as a refrigerant leak. Common reasons include power failures, thermostat malfunctions, clogged filters, or faulty capacitors. Ignoring these problems can lead to costly repairs and discomfort, especially during hot weather.

In this article, we’ll break down the most common reasons why your AC unit is not turning on and provide practical solutions to get it running again. Keep reading to troubleshoot your system and learn when to call a professional.

AC Not Turning On: Common Causes and Solutions

1. Power Issues

if your AC unit isn’t turning on, start by checking the power supply. Electrical issues are a common cause.

Check the Circuit Breaker

Look at your home's electrical panel and see if the breaker for the AC is tripped. If it is, reset it. If it trips again, this could indicate a serious electrical problem, and you should call a professional.

Inspect the AC Power Switch

Ensure the power switch near your indoor HVAC unit (such as the furnace or air handler) is in the "ON" position. For outdoor units, check the disconnect box and see if the fuse is blown. A technician may need to replace it.

Examine the Thermostat

A dead thermostat can prevent your AC from turning on. Replace the batteries if needed, set the mode to "Cool," and lower the temperature below the current room temperature. If it’s a smart thermostat, try resetting it or check for software updates.

Test the Outdoor Power Supply

For outdoor units, check for a safety shut-off switch and ensure it’s not turned off. Also, verify that the power cord is securely plugged in, especially for window or portable AC units.

2. Thermostat Problems

A malfunctioning or improperly set thermostat is a common reason why your AC is not turning on. Since the thermostat controls when and how your air conditioner operates, even small issues can prevent your system from cooling your home.

How Thermostat Issues Affect Your AC

If your AC unit is not turning on, the thermostat might be:

Set incorrectly – If it's in "Heat" mode instead of "Cool," your AC won’t start.
Battery-powered and dead – Some models stop sending signals when batteries run out.
Misconfigured – Smart thermostats may have energy-saving settings that delay or prevent activation.
Malfunctioning – A faulty thermostat may fail to communicate with your AC system.

How to Troubleshoot Your Thermostat

Follow these steps to ensure your thermostat isn’t the issue:

Check the Display – If the screen is blank or unresponsive, replace the batteries or reset the unit.
Verify the Settings – Ensure the thermostat is set to "Cool" and the temperature is at least 5°F lower than the current room temperature.
Inspect the Wiring – If you recently installed or moved the thermostat, a loose wire may be preventing it from working correctly.
Test Manual Controls – If using a smart thermostat, try switching to manual mode to see if it responds.
Reset the Thermostat – Some models have a reset button, while others require you to turn off the power for a few minutes before restarting.

3. Clogged Air Filters

A dirty or clogged air filter is one of the most common reasons why an AC unit is not turning on or failing to cool properly. When the filter gets too dirty, it blocks airflow, causing the system to overheat and shut down as a safety measure.

dirty air filter

How Dirty Filters Affect Your AC

Restricted Airflow – A clogged filter prevents air from moving freely, making the AC work harder and reducing efficiency.
Overheating & Shutdowns – Limited airflow can cause the evaporator coils to freeze, leading to system failure.
Increased Energy Costs – A blocked filter forces the AC to consume more energy, increasing electricity bills.
Poor Indoor Air Quality – Dust, pollen, and allergens accumulate, negatively impacting air quality and health.

How Often Should You Change Your AC Filter?

Every 1-3 months for standard disposable filters (depending on usage and household conditions).
Monthly if you have pets, allergies, or live in a high-dust area.
Every 6-12 months for high-efficiency HEPA filters (check manufacturer recommendations).

If your AC is not turning on, checking and replacing the air filter should be one of the first troubleshooting steps. Regular AC tune-ups and filter replacements can extend the lifespan of your system and ensure optimal performance.

4. Refrigerant Leaks

A refrigerant leak is a serious issue that can prevent your AC unit from turning on or cooling effectively. Refrigerant (also known as Freon or R-410A) is essential for absorbing heat and cooling the air. If levels drop too low, the system may shut down or run inefficiently.

Signs of a Refrigerant Leak

Warm Air from Vents – If your AC is running but not cooling, low refrigerant levels may be the cause.
Hissing or Bubbling Sounds – These noises near the indoor or outdoor unit indicate a possible leak in the refrigerant lines.
Frozen Evaporator Coils – Ice buildup on the coils suggests insufficient refrigerant, leading to poor cooling performance.
Higher Energy Bills – A refrigerant-deficient AC works harder, consuming more electricity.

How to Fix a Refrigerant Leak

Unlike clogged filters or thermostat issues, refrigerant leaks cannot be fixed as a DIY repair. Here’s what you should do:

Turn Off Your AC – Running an AC with low refrigerant can damage the compressor, leading to expensive repairs.
Check for Visible Signs – Look for ice buildup on the evaporator coils or listen for hissing sounds near the refrigerant lines.
Call an HVAC Professional – Only a licensed technician can:
- Locate and seal the leak using specialized equipment.
- Refill the refrigerant to manufacturer-recommended levels.
- Ensure your system is operating efficiently after the repair.

Is your AC not working? Call us today at (669) 322-8182 or contact us online for fast and professional HVAC services.

5. Frozen Evaporator Coils

Frozen evaporator coils are another common issue that can cause your AC unit not to turn on or lead to poor cooling performance. When the evaporator coils freeze, they are unable to absorb heat effectively, resulting in warm air circulating through your home instead of cool air.

Why Do Evaporator Coils Freeze?

Several factors can cause the evaporator coils to freeze:

Low Refrigerant Levels – As mentioned earlier, a refrigerant leak can cause the system to run low on coolant, resulting in freezing coils.
Poor Airflow – Blocked or clogged air filters, obstructed ducts, or malfunctioning fans can restrict airflow over the evaporator coils, causing them to freeze.
Dirty Coils – Dirt and debris can build up on the coils, reducing their ability to absorb heat and causing freezing.
Thermostat Malfunctions – If the thermostat is set too low or is malfunctioning, it can cause the system to run too long, leading to coil freezing.
Faulty Blower Fans – A malfunctioning blower fan may not circulate air properly, causing the evaporator coils to freeze.

How to Fix Frozen Evaporator Coils

If you notice your AC is blowing warm air or your evaporator coils are frozen, follow these steps:

Turn Off the AC – Allow the coils to thaw by switching off the AC. This may take several hours.
Inspect the Air Filter – Check if the filter is clogged. If so, replace it to improve airflow.
Check for Refrigerant Leaks – Low refrigerant levels can cause freezing, so inspect the system for leaks.
Ensure Proper Airflow – Make sure that vents, ducts, and registers are open and unobstructed.
Clean the Coils – If the coils are dirty, gently clean them with a soft brush or cloth. However, it’s best to leave this to a professional to avoid damaging the coils.
Call a Professional – If the issue persists after performing these checks, or if you suspect a refrigerant leak, contact an HVAC professional for further diagnosis and repair. Frozen coils often indicate an underlying problem, such as low refrigerant, a malfunctioning fan, or dirty coils that require expert attention.

6. Issues with the AC Capacitor

The AC capacitor is a vital component in your air conditioning system, responsible for providing the necessary electrical charge to start the compressor and fan motors. When the capacitor fails, the AC may not turn on or may struggle to operate effectively, leading to poor performance or complete system failure.

Signs of a Faulty Capacitor

AC is not turning on – The system may try to start but fail to power on.
Humming Noise – A humming sound without the system turning on often indicates a failed capacitor.
Intermittent Cooling – If your AC is running but cooling erratically or inconsistently, the capacitor may be malfunctioning.
System Shuts Off Suddenly – If the system turns off abruptly without warning, it may be due to a capacitor issue.
Blower Fan or Compressor Not Running – If one or both of these components are not working, the capacitor may be the cause.

How to Fix AC Capacitor Issues

If you suspect that the capacitor is the problem, here’s what you can do:

Turn Off the Power – Always disconnect the power to your AC before inspecting or attempting any repairs.
Inspect the Capacitor – If you feel comfortable doing so, check the capacitor for signs of damage, such as bulging, leaking, or scorch marks. However, this requires caution, as capacitors store electrical energy.
Test the Capacitor – A professional can use a multimeter to test the capacitor for proper voltage and functionality.
Replace the Capacitor – Capacitors are typically inexpensive and can be replaced by a technician if damaged.
Contact a Professional – If you notice any issues with your capacitor or suspect it may be malfunctioning, it’s best to call a certified HVAC technician to handle the repair. Replacing a faulty capacitor requires technical expertise, and improper handling can lead to further damage or injury. A professional will ensure the correct capacitor is installed and that your AC system runs smoothly.

Central AC Not Working? We Can Help

In this article, we've covered some of the most common issues that can cause your AC to stop working and how to troubleshoot them. While some problems can be fixed on your own, others are more complex and require the expertise of a professional.

If you're dealing with persistent issues, don’t risk further damage by attempting to fix them yourself. At IRBIS HVAC, we’re here to provide expert diagnosis and repair services for all HVAC-related problems.

Serving San Jose, Redwood City, Cupertino, Sunnyvale, Santa Clara, and the surrounding Bay Area, our team is ready to help get your AC system back in top shape.

Need fast and reliable AC repair?

Call us now at (669) 322-8182 or schedule a service online. We’ll get your AC working properly again, so you can stay comfortable in your home.

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