Car AC Pressure When Off – Causes and Fixes

Air conditioning systems are very delicate and can develop various issues, including high car ac pressure when off or fluctuations in pressure. Some drivers do not pay attention to these issues because it doesn’t affect their vehicles’ performance.

But leaving your ac system to deteriorate isn’t such a good idea; it’s always best to tackle the problems whenever they surface. Besides, it can be frustrating driving on a sunny day, and you cannot use your air conditioner.

If you know the causes of the problem and how to fix them, you might not have to go to a mechanic’s shop for simple fixes.

What causes car ac pressure when off?

When your ac pressure is higher or lower than it should be, even when your engine is off, it implies a few problems in your vehicle’s system. But make sure you know how to check ac pressure in a car, so you don’t get a false reading.

car ac compressor shuts off after a few seconds

An overcharged air conditioning system

The reason you are experiencing high ac pressure when your engine isn’t running can be due to an overcharged system, which implies that your entire conditioning system is overcharged.

This can be due to the presence of a high amount of refrigerant in your system or high oil quantity, and as a result, it causes built-up pressure in the system.

You have to release this pressure somehow, but you should allow your mechanic to do that for you.

Problems with the condenser fan

You can have dirt or debris accumulated in your system over time, and this debris blocks the airflow that comes through the condenser fan motor.

A blockage in your system will reduce airflow through your condenser, making the air conditioning system unable to cool down.

If your condenser fan functions at very high pressure, it can damage the aluminum, then you begin to experience leakage from the evaporator coil.

When this happens, it can destroy your whole air conditioning unit. It’s also very possible that the condenser fan’s motor is faulty.

Faulty condenser

The condenser of an air conditioner is mainly for absorbing high pressure and temperature from your compressor. A condenser is designed to eliminate the heat and reduce the temperature of the freon or refrigerant.

If you want to know what causes low side ac pressure too high, you should probably check your condenser.

A faulty or damaged condenser will not manage the pressure generated by the air conditioner, which can cause high pressure in your ac system, even after you’ve put off your engine.

To really know the cause of this problem, you have to test your cooling system when your engine is ON.

But if you have a high ac pressure when your engine is still running, it is normal.

It is quite possible air could have gotten inside your coolant system, increasing the pressure.

Read Also: How to Jump AC Compressor Clutch Correctly

How do you fix car ac pressure when off?

To get the problem fixed, you have to inspect your air conditioning system so that you can determine the particular cause or causes of the problem.

what is the correct ac pressure on a car

If you’ve discovered debris in your system, you have to flush it properly. But to remove dirt or oils from your system, you are to detach the expansion valve, drier, and compressor.

Also, suppose you want to retrofit your system from R-12, you have to flush out the old oil. Mixing refrigerants or oils can damage your system.

When you’ve reached your expansion valve and have taken out the evaporator from the car, you might discover it’s very dirty.

You can gently use water to wash it off but ensure that you don’t get water into the pipes that transport the freon. Replacing the expansion valve is an option you should consider.

Wrap the valve in a thermo-insulation material, using either glue or other packaging material.

Now for the flushing process, you just have to spray your ac flushing compound into the pipes.

Wait about 20 mins and flush the compound out with a shop air; a shop compressor is preferable. If you use the wrong compressor, it might not be able to do a good job.

If your compressor leaks freon from the O-ring seal or makes noises, it shows it’s time you replace the compressor.

It’s actually a straightforward process. Ensure you apply oil to your suction side and turn the clutch around about 15 to 20 times, so the oil is cleared.

When fixing your new compressor, make sure you tighten the mounting bolts evenly not to affect your ac compressor pressure or case.

While fixing new seals for your freon pipe, rub oil on the surfaces till they are properly lubricated. If you want to refurbish your compressor so you can still use it, wrap the compressor in some newspapers so you will be able to reuse the box.

Read Also: Why Is My Dashboard Air Vents Not Working?


Q: Why does AC only work when my car is moving?

When your vehicle is moving, air flows more over the condenser (i.e., the part responsible for heat exchange located before the radiator), and airflow is very important for proper system operation.

Other reasons might be that your compressor clutch has rotation problems as intended, or maybe your condenser is blocked or dirty.

Your car’s AC might be suffering from poor maintenance also; you have to check your AC system for issues properly. If your fan is damaged and cannot operate with full force, it will not be able to cool down the heat from the Freon (i.e., refrigerant) via the condenser.

When your vehicle is at high speed, your condenser does not need the fan to cool down the Freon because enough air is passing through it, but without moving, it is fully responsible for this action.

If your engine is overheated, the AC may not function when the car is not moving.

Q: Do you check AC pressure with a car running?

No, you shouldn’t do this at all because you won’t get the right reading.

Take your measurement when the pressure in a car when the engine is off, and the AC has not been used for at least an hour, the pressure measured (both high and low) will equalize, so you get your correct reading.

You can use an ac pressure chart to know if you have a good reading.

Q: Should my car AC compressor cycle on and off?

It is completely normal for your vehicle’s compressor cycle to on and off. But when this becomes too frequent, it may be due to an improper charge (for example, being low on Freon).

If AC is fully charged, this will not be a problem. Most cars in old model products were equipped with electronic modules that tell the vehicle’s AC compressor when to cycle on or off.

Although this might alarm some people, it is completely normal and often occurs when idling.

Compressors cycle also to be able to prevent your vehicle’s internal pressure from going above some maximum pressures found in the system. If this is functioning properly, the AC compressor will stay on for about 45 seconds, switch off briefly, and come back on.

On hot days, people set their AC to cooler states; the more your higher the pressure on your compressor to maintain this cooler temperature.

Q: What are normal pressures for 134a?

Few ask what is the correct ac pressure on a car for 134a. The best running pressure for R134A is found between 22 and 57 in this type of system. R134A is proven not to be explosive with ambient temperature and un-corrosive to metals, e.g., copper, aluminum, and stainless steel.

The refrigerant pressure for R134A is determined at any temperature found between -22-2020 Fahrenheit read from a valid pressure temperature chart.

For the lowest temperature, the accurate running pressure of this coil is found between twenty-two pounds. But the highest temperature, it runs out fifty-seven pounds.

Q: Does AutoZone check Freon?

When you need to check your AC recharge, AutoZone will always be helpful; they use R134a refrigerant, AC stop leak, PAG46 oil, AC system cleaner, and so much more.

This process can either be done online or by visiting close by shops. AutoZone also helps test your vehicle’s parts for free, e.g., your alternator, battery, voltage, and starter.

Q: Will car AC work if overcharged?

When your AC is overcharged, it can lead to the car having serious issues without proper attention given to it. This might lead to your AC not working at all or very weak.

It can also lead to AC system failure or damage to your AC condenser. When AC is overcharged, it leads to high pressure and temperature within the system.

One of the symptoms of overcharged AC is a poor cooling system. If the system is overcharged frequently without enough room to depressurize, your AC system will stop functioning altogether.

The compressor will start making disturbing noises, and it will read high pressure, and your engine will struggle.

However, check your pressure with your ac static pressure chart to know if you are OK in that area.

Q: Can I diagnose my ac charge level in my car with gauges using static pressure?

A lot of sources advise you to check your ac charge through static pressure. Well, it’s a good first step, but you can’t rely on the readings of static pressure as the only indication of charge determinant.

Measure your pressure readings when your engine is off, and your ac hasn’t been on for at least 50 mins. If not, it will cause your car ac pressure to not equalizing.

Static pressure readings only indicate when your system is empty or has refrigerant. It can’t be used to tell if you have the correct charge.

Final Words

Servicing an air conditioner can be very technical, and it might require a professional’s touch with the right tools. Fort most services, you can’t just do a quick one and get out of there.

If you are certain there’s something wrong with your car ac pressure when off; a better decision is to get your car to a professional like us and get your car fixed rightly.

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Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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