13 Reasons Why Your AC Won’t Turn On and How to Fix Them

Is your AC not turning on and you don’t know what to do? We understand. There are many parts that could be causing the problem. It might be the thermostat, the cable, the compressor, or something else.

But don’t worry. In this guide, we’ll explain all the possible causes and solutions. That way, you’ll know if you can fix it yourself or if you should call air conditioning services. Let’s get into it.

13 Reasons Why AC Won’t Turn On and How to Fix Them

These are the reasons why your AC is not turning on and how to fix it properly:

1. Thermostat Set to Wrong Mode

Your AC isn’t turning on, and you worry it’s a major technical problem. You might think about costly repairs or replacement.

But often, the issue is simply that your thermostat is set incorrectly. This is common after a power outage or if someone bumped the settings.

How to fix the AC not turning on when this happens:

1. Check your thermostat display.
2. Find the mode button or dial.
3. Note the current mode (Heat, Cool, Auto, Off).
4. Switch to “Cool” mode.
5. Set the temperature lower than the room temperature.

2. Thermostat Batteries Dead

Your air conditioner won’t turn on, and this time the thermostat screen is blank. So, what’s going on? It could be the batteries.

Your thermostat uses AA or AAA batteries to power the display and communicate with your AC. When these batteries are drained, there are no lights and no settings.

How to fix the AC not turning on when this happens:

1. Find the thermostat’s battery compartment on the back or side.
2. Open it and identify the battery type.
3. Remove old batteries and insert new batteries.
4. Close the compartment and wait for the display to light up.
5. Set the thermostat to “Cool”.

3. Thermostat Miscalibrated

The cool mode is on, and the batteries seem fine, but your thermostat still isn’t responding. This time, the issue might be with the thermostat itself. It may be broken or miscalibrated.

A miscalibrated thermostat doesn’t read the actual room temperature accurately. A faulty thermostat may also fail to communicate with your system altogether.

How to fix the AC not turning on when this happens:

1. Check your thermostat’s manual or settings for a “recalibrate” or “reset” option.
2. Follow the recalibration steps if available and wait for the process.
3. If no recalibration option or it doesn’t work, the thermostat likely needs replacing.
4. Contact an HVAC professional to install a new thermostat and verify wiring.

4. Thermostat Sensor Dirty or Blocked

Your thermostat settings are right, the batteries are new, but your air conditioner still won’t turn on?

Remember, your thermostat has a small sensor that checks the room’s actual temperature. If dust covers it, the sensor gives wrong readings and might not turn on the cooling.

Not to mention that windows in direct sunlight can make it think your house is warmer than it really is.

How to fix the AC not turning on when this happens:

1. Remove your thermostat’s cover.
2. Find the small dark spot or metallic probe. That’s your sensor.
3. Clean around it with a soft, dry cloth or small brush.
4. Place your thermostat on an interior wall away from direct sunlight.
5. Wait at least 5 minutes to see if the AC turns on correctly.

5. Clogged Air Filter

Your air filter keeps your AC clean and indoor air fresh. But a dirty filter blocks airflow, making your AC work harder. This strains the motor and can cause shutdowns, as coils freeze or the system overheats.

How to fix the AC not turning on when this happens:

1. Find your air filter, usually in the return vent, or in the duct.
2. Take out the filter and check if it’s very dirty.
3. If it can be reused, vacuum it or rinse it with water and let it dry completely.
4. For disposable AC filters, replace it with a new one that is the same size and rating.

6. Condenser Coils Dirty

The outdoor condenser coils are fins on your AC unit that let heat escape from inside. In dusty environments, they gather dirt and grass, which insulates the coils.

If the blockage is severe, your system may overheat and shut down, potentially causing damage to components. So, you need to clean the AC coils.

How to fix the AC not turning on when this happens:

1. Turn off your air conditioner at the thermostat.
2. Check the condenser coils outside.
3. Rinse the coils gently with a low-pressure garden hose from inside out.
4. Turn the disconnect back on, set your thermostat to “Cool,” and check performance in 30–60 minutes.

7. Circuit Breaker Tripped

Your AC system has its own circuit breaker in your electrical panel to protect against power surges or overloads. When there are problems, the breaker trips automatically to cut off the power.

How to fix the AC not turning on when this happens:

1. Find your home’s electrical panel.
2. Open the door and locate the breaker labeled “AC,” “Air Conditioning,” or “Cooling.”
3. Check if the switch is in the middle or toward “Off.”
4. If tripped, push it firmly to “OFF”, then to “ON.”
5. If the breaker trips again immediately, stop and call a professional HVAC technician.

8. Power Cord Damaged

In split systems installation, your outdoor AC unit and indoor components rely on constant power from a cord plugged into a dedicated outlet.

Over time, cords can become damaged from weather or wear. Outlets may also stop working internally due to rust or electrical issues.

How to fix the AC not turning on when this happens:

1. Check the AC power cord for damage, cracks, or burn marks.
2. Look at the outlet for damage, discoloration, or scorch marks.
3. If the cord is damaged or you think there are faulty outlets, call an electrician.

9. Broken Capacitor

You’ve tried all DIY troubleshooting steps, but nothing has worked. At this point, you’re likely dealing with an internal component failure. And a common cause is a broken AC capacitor.

When a capacitor fails, it can’t supply startup energy. So, even if power reaches your AC unit, the compressor and fans won’t turn on.

How to fix the AC not turning on when this happens:

1. Listen to your outdoor unit, do you hear a faint hum but no compressor?
2. If yes, a broken capacitor is the problem. Call a licensed HVAC technician to check and replace it.

10. Low Refrigerant

Your AC unit won’t turn on, even though all the electrical and mechanical parts are working. You should check the refrigerant level. Without enough refrigerant, your AC can’t cool properly.

Leaks often happen over time from rusted coils, loose fittings, worn-out seals, or bad installation. Small leaks can go unnoticed for years until the pressure becomes too low.

How to fix the AC not turning on when this happens:

1. You can only look for signs like warm air, hissing sounds, or ice on the evaporator coils.
2. Don’t fix it yourself, just call your technician with refrigerant handling certification.

11. Control Board Failed

If you’ve learned how AC systems work, you know the control board is basically the brain of your AC. It takes signals from your thermostat and controls everything, from compressor and fan motors to capacitors.

When the control board burns out or has internal faults, it can’t send the startup commands to your compressor.

How to fix the AC not turning on when this happens:

1. Check the indoor AC control board for melted plastic or damage.
2. Contact a technician to diagnose and replace the board.

12. Broken Fan Motor

Your AC won’t turn on, and when you go outside, you see that the fan on your outdoor unit isn’t spinning.

Remember that if the outdoor fan motor isn’t working, hot air can’t get out, which makes your AC not blowing cold air or not turn on at all if a safety shutoff is triggered.

How to fix the AC not turning on when this happens:

1. Look at your outdoor unit while the AC is on “Cool” and see if the fan blades are turning smoothly.
2. If the fan isn’t turning but the compressor runs, the fan motor likely stopped working.
3. Contact a pro technician to replace the outdoor fan motor.

13. Compressor Mechanical Failure

This is the worst-case scenario when troubleshooting your AC. The compressor is one of the most important AC unit parts.

It is the most expensive and hardest-working component in your entire air conditioning system. When it fails, it can’t pump refrigerant, and your AC won’t cool at all.

How to fix the AC not turning on when this happens:

1. Listen for strange grinding, squealing, or knocking sounds from your outdoor unit.
2. Call your professional technician to find out what’s wrong and to replace the part.
3. If your unit is over 10 years old, think about replacing the entire AC.

How to Prevent Your AC from Not Turning On

You know how to fix your AC when it won’t turn on. But these maintenance tips can help you avoid common problems:

1. Change Air Filters Regularly

Change your disposable filters or clean your reusable ones every 30–90 days, depending on dust, pets, and air quality. A dirty filter blocks airflow, making your AC work harder and putting strain on its parts.

2. Clean Your Outdoor Unit

Remove leaves, sticks, dirt, grass, and trash from around and inside your outdoor condenser, especially after storms. Keep at least 60 centimetres of space around it to ensure proper airflow and reduce strain.

3. Check Your Ductwork

You can check visible ducts for leaks, collapses, or disconnections that cut cooling. Catching these early can prevent more frequent ducted air conditioning repair.

But still, for major damage or suspected blockages inside walls or attics, you need to call a professional.

4. Book a Professional Check-Up

Hire a licensed HVAC technician at least once before spring’s cooling season. Ideally, schedule a second visit after summer’s heavy use. Regular check-ups find problems early like refrigerant leaks or electrical issues.

When Should You Seek Professional Help?

In short, if the problem doesn’t involve electricity, refrigerant, motors, or internal parts, you can fix it yourself. If it does, stop right away and call a professional.

So, when your AC won’t turn on after checking the thermostat, replacing batteries, and clearing vents, that’s when professional technicians from Lightning Bult can step in.

These professionals can take care of electrical diagnostics, refrigerant recharges, motor and compressor repairs, and internal component issues—tasks that require expertise and certification.

FAQ About AC Unit Not Turning On

These are some of the most common questions homeowners ask about their AC not turning on:

Why is my AC light not turning on?

A blank thermostat display often signals dead batteries or a tripped breaker. So, replace the batteries first. If it stays dark, check your main panel for a breaker labelled “AC” or “Cooling” and reset it.

How do I reset my air conditioner?

You can turn off the outdoor AC disconnect switch or circuit breaker. Then, wait 5-10 minutes. After that, turn it back on. This full power cycle resets the system’s control board without causing damage.

How do I force start an AC?

No. Don’t force your AC to start. It can cause permanent damage. What you need to do is to check the thermostat, battery, circuit breaker, air filter, and outdoor unit.

If it still won’t turn on, call a professional. Forcing it risks costly damage.

Does resetting an AC unit help?

Yes, resetting clears electrical glitches, frozen compressors, and stuck contactors. It fixes temporary faults without erasing settings.

If problems persist, the issue is likely mechanical or electrical and requires professional repair.

What is the first thing to check when the AC stops working?

You can check your thermostat. Make sure it’s on “Cool” and at least 5 degrees below room temperature. If the display is blank, replace the batteries.

Conclusion

Is it the thermostat settings or the clogged air filter? You know all the possible causes when your air conditioner isn’t turning on and how to fix it.

Just remember, know your limits. Some fixes are safe to do yourself, but others require a licence just to start checking.

If you’re unsure about any repair, don’t risk it. Contact Lightning Bult today. We can diagnose your air conditioning issue, perform repairs safely, and get your system running again.