What Should AC Pressure Be When Car Is Off?

When driving in the hot summer, the last thing you want is for your car's air conditioner to fail. But when it happens, you will need to identify the problem before you can fix it, and one way to do this is by checking the AC pressure.

So, how can you tell what the correct pressure should be? First, let us take you to what we have researched in this post.

The standard reading for AC pressure when a car is not running is 25–40 PSI for the low side and above 200 PSI for the high side.

The pressure may increase or decrease depending on the surrounding temperature, ideally 70–80 °F. However, your AC may not function well if you notice an abnormal reading between the low and high sides.

Getting the correct AC pressure should be one of the few things car owners should know how to do by themselves. It is essential for you to immediately diagnose problems arising from getting abnormal readings.

This post will take you step-by-step through learning how to read AC pressure correctly and what problems it may be causing.

Car air conditioner check service, leak detection, fill refrigerant.Device and meter liquid cooling in the car by specialist technicians - What Should AC Pressure Be When Car Is Off

AC Readings And Issues

A working AC system has an ideal pressure gauge of 25-40 PSI on the low side and higher than 200 PSI on the high side when your car is off.

A reading below or above this range could mean one thing—a broken AC system.

Here is a list that shows abnormal results you may encounter as you check your car's AC pressure and their possible causes:

Low Side / High Side

  • 125 PSI / 30 PSI - Too much oil in the compressor or an inadequately charged system. 
  • 150 PSI / >10 PSI - A leak or expansion valve stuck open.
  • 150 PSI / 30 PSI - Presence of water in the system.
  • 160 PSI / 10 PSI - A frosted evaporator, faulty low-pressure piping, or clogging expansion valve.
  • 200 PSI / 70 PSI - System blockage before or at the expansion device. 
  • 225 PSI / 80 PSI - Widely-opened expansion valve or too much refrigerant.
  • 250 PSI / 30 PSI - Presence of air in the system. 
  • 250 PSI / 50 PSI - Overcharged system, AC not cooling, blocked condenser.

Having learned these various readings and their possible causes, let us review how to correctly check your car's AC pressures.

Guide To Checking The AC Pressure Of Your Car

In checking the AC pressure of your car, start by following the simple steps we have listed below: 

Preparing Your Equipment

1. Prepare the manifold gauge set. Ensure both valves are closed before starting, and then hang the hose on the hood. 

2. Locate the service ports on your car. The red cap is for the high port, and the blue cap is for the low port. Remember to use safety glasses and gloves before tapping the hose to the valves.

3. Get inside your car and take a thermometer with you. Place the thermometer on the center vent to measure the air's output. A good number will be a 45-degree reading. 

Watch this video for a clear illustration of the steps to getting an accurate AC pressure reading:

Switching On The Engine

1. Turn the AC fan to the 3rd setting while turning on the sync button to maximize the airflow inside your car.

2. You should get an accurate reading after you get the car to warm up in idle mode. Do this by increasing the RPM to 2,000 (depending on the car model) by inserting an unused credit card under the throttle body. 

3. Now, check your pressure gauge and refer to the chart we have provided below:

man hand turning on car air conditioning system

Check out this AC refrigeration kit on Amazon. 

Basic Troubleshooting Tips

Here is a list of common scenarios and how you can troubleshoot them.

Low Side Pressure—High: High Side Pressure—Low

1. Too much Freon going into the evaporator.

2. A thermal bulb failing to read temperature signals.

3. The thermal expansion valve is stuck to the open position.

4. The orifice tube's opening has gone more extensive than the usual size, which lets more refrigerant get into the evaporator; this happens the same when the O-ring seal is wearing out of the tube. 

How To Solve This Issue

Car air conditioner check service, leak detection, fill refrigerant.Device and meter liquid cooling in the car by specialist technicians.

1. Check the pressure switch located just near the expansion valve.

2. Confirm if the expansion valve is giving the correct reading using an Ohmmeter.

3. Lastly, look at the compressor clutch if it is engaging and clutching while the AC is on—you will see that the fan also spins when the system is working. 

If you still have the same reading after checking, consider changing the compressor to fix the unit. 

You can check out this Ohmmeter on Amazon. 

Low Side Pressure—Low: High Side Pressure—High

1. The condensing fan is defective and fails to manage the pressure of the AC.

2. An overcharged AC causes built-up pressure because of high refrigerant or oil quantity.

How To Solve This Issue

You can release the pressure to fix this, but we recommend asking a mechanic.

Low Pressure On Both Sides 

Warm air is blowing from the vents; this is caused by a low refrigerant in the system, decreasing oil circulation in the compressor.

How To Solve This Issue

Lubricating the AC compressor with an oil mist contained in the Freon mixture. 

High Pressure On Both Sides

According to an automotive mechanic, an obstruction in the evaporator can usually cause this reading.

In addition, debris, like dry leaves and dust, may have contributed to this, or worse, it may be due to internal damage. 

How To Solve This Issue

Removing blockage and clearing debris should do the job of restricting airflow and then rechecking the readings for confirmation. 

What To Do When You Get Equal Readings On Both Sides

A 50/50 PSI reading could mean that the clutch is not engaging, while a 100 PSI /100 PSI reading could mean that the compressor is not pumping because of a burnt coil.

You can confirm this by turning off the engine, rotating the center hub by hand, and checking for resistance. 

What Causes Car AC Pressure When Off?

A high amount of refrigerant and oil quantity can cause an overcharged conditioning system because of the built-up pressure in the conditioning system.

This results in the inability of the refrigerant to switch between a gaseous into a liquid state.

Also, there could be airflow blockage in the motors that are causing condensing fan problems.

Why Is It Important To Maintain The Correct AC Pressure While The Car Is Off?

Car air conditioner check service, leak detection, fill refrigerant.Device and meter liquid cooling in the car by specialist technicians.

It is essential to avoid expensive repairs since the car's AC system works by circulating a refrigerant through a series of valves, coils, and hoses.

If the system's charging is inadequate, the refrigerant will leak, and the AC cannot cool the car effectively.

Low pressure can cause the compressor to overheat and fail, and high pressure can damage the compressor seals and cause coolant leaks. 

What Happens When AC Pressure Is Too High? 

When AC pressure is too high, it can make the AC less effective and damage the compressor. It can also decrease cooling capacity, freeze the evaporator, and cause compressor failure. At worst, it can cause the car's engine to overheat.

How To Adjust The AC Pressure If Necessary

You can easily adjust the pressure with a simple gauge. Connect the indicator to the low-pressure port and turn on the AC.

The needle should rest in the gauge's green section. If it doesn't, adjust accordingly.

How Does Car AC Work When The Engine Is Off?

According to a forum response, car AC doesn't work while off because it needs a compressor to act on the refrigerant.

In addition, the required power for an AC needs an engine to run for the belt to start turning and power the whole system. 

How Do You Easily Spot Abnormal Pressures In AC? 

Your AC system's problem can be detected when any of these conditions exist:

  • AC is not giving cold air.
  • Vents won't release airflow. 
  • Only cool air is emitted, but it doesn't get cold. 
  • A mildew smell from the AC.
  • AC goes from cold to hot. 

Bonus Tips For Maintaining Your AC System

Driver hand tuning air ventilation grille, fresh air is coming out

To get the most out of your car AC this summer, you can check out these simple tips we found on how to maintain them:

1. Deep clean your car once a year.

2. Changing the air filter regularly to protect the car from becoming ground for bacteria caused by clogged dirt and trapped moisture.

3. Use your AC correctly every time you turn on the engine: Open the windows halfway down and turn the fan to the maximum setting first so it will blow away hot air from the inside and cool the vehicle's interior.

4. Defrost your car at least once a week. To do so, turn on your AC to its coolest setting and highest fan speed for about 10 minutes and defrost it for 5 minutes to clean out the moisture from the system to prevent mildew.

5. Avoid pre-cooling your car before it starts since the AC system cools things more when there's airflow. Turn on the AC only once the vehicle starts running.

6. Park under the shade to avoid the car from becoming too hot, making the AC system work hard to cool the interior.


When it comes to AC pressure, the best thing you can do is consult your car's manual. Every vehicle is different and will require an extra level of pressure when the engine isn't running.

Keeping your vehicle at the correct pressure will help ensure that your system works as efficiently as possible and that you don't run into any problems down the road. 

We hope this article has helped you, but before you go, you may check out these related articles:

Car AC Only Works When Driving – What's Wrong?

How To Know If a Car Air Conditioner Needs Freon 

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