AC Low Side Pressure Too High? Here’s Why

Is your AC low side pressure getting too high? Why is it happening and what should you do? Here's a super simple guide.

Who doesn’t love blasting their car’s AC on a hot summer day? Unfortunately, if AC pressure is too high on the “low” side of your vehicle’s AC system, cold air is unlikely to come out of the vents.

What causes high pressure on the low side of an AC system?

There are six possible culprits for high pressure on low-side AC, including:

  1. Defective condenser fan
  2. Faulty AC compressor
  3. Clogged condenser
  4. Contaminated AC coils
  5. Bad expansion valve
  6. Overcharged AC

In this guide, I’ll share more details about each of the above causes and teach you how to test for a high-pressure-on-low-side AC system.

I’ll also answer some frequently asked questions about AC high low-side pressure, including:

  • What are the symptoms of a faulty AC system?
  • How often should I replace the AC compressor in my car?
  • Can I drive with high low-side pressure in my AC system?
  • How much does it cost to repair a faulty AC system?

Let’s get started.

Table of ContentsShow

AC Low-Side Pressure Too High? Understanding Car AC Systems

Does A Car AC Use Gas

A vehicle’s AC system is complicated, right up there with the combustion process, transmission, torque converter, and more.

Simply put, turning your car’s AC on results in refrigerant cycling through a loop that’s divided into low and high-pressure sides. It starts as a liquid, converts to gas, and ends up back as a liquid. Regardless, by the time it has gone through the entire loop, cold air should be coming out of the vents.

As for the low and high sides, the low side of a car’s AC system handles drawing in refrigerant, while the high side compresses and distributes it.

The low side works under lower pressure and temp, while the high side has a higher pressure and temp. Low-side components include the compressor, evaporator, accumulator, and expansion valve. The high side consists of the condenser, receiver dryer, and expansion valve.

For a full rundown on how a car’s AC system works, check out this guide I wrote a while back.

6 Reasons Why Your Car’s Low-Side Pressure Might Be High

1. Bad Condenser Fan

Car AC Condenser Fan

A bad condenser fan can cause low-side pressure to be high, preventing the AC system from cooling properly. The fan blows air over the condenser, a part responsible for releasing heat absorbed by the refrigerant.

If the fan isn’t working properly or is not spinning at all, the condenser won’t release the heat efficiently, causing high pressure on the low side. This can stem from a faulty fan motor, damaged blades, or electrical issues.

2. Faulty AC Compressor

Car AC Compressor

A faulty AC compressor can also cause low-side pressure in your car’s AC system to be high. The compressor’s job is to compress the refrigerant and send it to the condenser for cooling.

If the compressor is faulty, you’ll likely experience high pressure on the low side of your AC system. This issue may require a compressor replacement or repair. Possible symptoms of a faulty AC compressor include strange noises, poor cooling, and visible leaks.

3. Clogged Condenser

car ac condenser

The condenser, a heat exchanger, removes heat from the refrigerant and releases it outside. Dirt, and other debris, like insects, can collect on the condenser’s fins, reducing its effectiveness. As a result, the refrigerant won’t be able to cool enough, causing high low-side pressure.

A clogged condenser can also cause high low-side pressure in your car’s AC system. Routinely cleaning the condenser and combing the fins can help protect against this issue.

4. Contaminated AC Coils

Yet another cause of high pressure on low-side air conditioning is contaminated AC coils. AC coils oversee evaporating the refrigerant and absorbing heat from the air inside the car.

If contaminated, the coils won’t be able to absorb efficiently, leading to high pressure on the low side. This issue can be resolved by cleaning or replacing the AC coils. Routinely maintaining your AC system can help prevent this problem from occurring.

5. Malfunctioning Expansion Valve

Car AC Expansion Valve

Your car’s expansion valve regulates refrigerant flow and controls how much flows into the evaporator. If it gets stuck open, too much refrigerant can flow in, causing the pressure on the low side to increase.

In contrast, if the valve is stuck closed, not enough refrigerant will flow through the system, leading to poor cooling. Replacing the expansion valve is often necessary to fix this issue.

6. Overcharged AC

Symptoms Of Overcharged Car AC And How To Fix

And finally, an overcharged AC may also be the reason for high low-side pressure. If too much refrigerant is added during a recharge or repair, it can cause high pressure on the low side. This can result in poor cooling performance and even lasting damage.

To resolve an overcharged AC, you’ll need to evacuate the system and recharge it with the correct amount of refrigerant. Your car’s owner’s manual will say exactly how much.

How To Test A Car’s AC Pressure At Home

Testing your car’s AC pressure yourself is easy and is a great way to save on repair costs. Just note that you’re dealing with high-pressure gas and liquid, so there are some potential risks. If you’re not mechanically savvy, you might do best to visit a professional.              

I’ll provide the basic steps below. If you prefer a visual guide, check out this video from YouTube’s Ratchets and Wrenches below.

As for what you’ll need, just a low-side gauge and a high-side gauge, both of which are available at most reputable auto parts stores. Both typically come with instructions on how to use them.

First, locate the low-side service port, which is usually found on the larger diameter hose coming from the compressor. Attach the low-side gauge to the port and start the car. Turn the AC to the highest setting and allow it to run for a few minutes.

Next, find the high-side service port, which can be found on the smaller hose coming from the compressor. Attach the high-side gauge to the port and read the pressure readings on both gauges.

On average, the low side should read between 20 to 40 PSI, while the high side should come in around 150 to 175 PSI. Compare your readings to your car’s manual to see if they’re within the manufacturer’s recommended range.

If the readings are outside of the recommended range, it’s best to consult a mechanic for further diagnosis and repair.

FAQ: AC High Low-Side Pressure

What Are The Symptoms Of A Faulty AC System? 

The symptoms of a faulty AC system include warm air blowing from the vents, unusual noises, foul odors, and weak airflow.

A drop in fuel mileage can also be a sign of AC system issues.

How Often Should I Replace The AC Compressor In My Car? 

There is no fixed interval for replacing an AC compressor. It depends on your car’s make and model, how often you use the AC, and how well you maintain it.

Regular servicing can help catch potential issues early.

Can I Drive With High Low-Side Pressure In My AC System?

It’s not recommended that you drive with high low-side pressure in your AC system.

Doing so can cause further damage to the system, reduce cooling efficiency, and potentially lead to system failure.

How Much Does It Cost To Repair A Faulty AC System?

Costs depend on the exact issue, the make and model of your car, and the cost of labor in your area.

Simple fixes like replacing a condenser fan can cost a few hundred dollars while replacing the compressor can cost several thousand.

Regularly Servicing Your Car’s AC Will Result In A Long And Problem-Free Experience

Routinely servicing your car’s AC system helps prevent high low-side pressure issues and keeps the AC running smoothly.

On average, you should service your AC system every two years. Regularly cleaning the condenser and replacing the AC coils helps prevent problems from occurring. Fixing any AC-related issues as soon as they arise also cuts down on overall repair costs.

By taking care of your car’s AC system, you can avoid problems and enjoy a comfortable and cool ride.

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Joshua Barrett

Josh Barrett is a writer hailing from the great state of Alaska. While describing himself in the third person is not his forte, writing about any and all things automotive – is. After 13+ years hustling in the exciting world of car sales, he took off to travel the world with his dog Teemo.