Why Is AC Pressure Too High? Causes And Solution


In the summer we all want a cool car, especially on a long drive. Sometimes the air conditioning seems like it's not working properly and I get semi cool air temperature. I wonder if my engine is too hot or is there another problem.

The air conditioning (AC) performance is important because it keeps the vehicle's interior temperature under control. If your AC pressure is persistently too high, it points to an underlying issue. When the pressure of the AC system is too high this might indicate a leakage or diminishing amount of refrigerant, both of which require immediate attention.

When I turn the AC on and it's warm, sometimes I turn up the fan hoping things will get cooler faster. If the AC temperatures are abnormally warm, this could indicate an issue with the compressor that might need repair or replacement. We'll discuss about how AC pressure can determine the health your AC system.

Importance of the AC Readings


Higher than normal temperature readings while testing your AC might indicate other problems as well. High temperature readings from your AC can indicate damage is occurring to your AC system. It needs to operate within a specific pressure range. High pressure might gradually lead to the failure of the compressor. If not corrected in time, the repairs can cost a fortune.

Problems with AC Pressure

High pressure inside the condenser fan can cause the aluminium to distort and evaporator coils to leak. The cost of repair and replacement of compressor is high (about $450 plus $700 labor - price fluctuates greatly depending on car) as the dash might need to be removed for repair. Higher than normal air pressure is usually a consequence of two situations.

One, your AC system might be facing limited or no airflow through the condenser. This, in turn, might be a consequence of bad condenser fan motor (Amazon Link). If you have debris near the fan blocking the flow, you will face the same issue. Second, the AC system could be overcharged with too much refrigerant.

Condenser Fan Problems


Photo Credit: http://rinconrolla98.blogspot.com/2015/03/diy-replacing-ac-condenser-fan-honda.html?spref=pi

The condenser fan of AC system works at different speeds and capacities. Hence, a malfunction might reduce its efficiency along with speed. This, in turn, leads to the inability of the fan to effectively cool the system and pass air.

Problems with the AC System

Overcharging of the system is usually caused due to an excess of refrigerant in the system. It can also be because of excess oil in the system. To test for an overcharge system, you would need to measure the pressure of the system. (Make it a point not adding more refrigerant to the AC system, unless you know for sure it is low.)

To avoid overcharging of the system, first check for leaks. Get a leak check kit or gas sniffer (Amazon Link) to find any underlying leakage. Do not allow any airline to be open as this causes the air to enter into the system, resulting in high-pressure readings. 

When checking for leaks, make sure the look at the lines, hoses, and compressor. You may need to replace one of these parts or just an O-ring. Do a visual inspection and look for oil leaking from any of these areas, and you'll know there's a leak.

Most of the vehicles currently use the R134a refrigerant (Amazon Link) in the AC systems. Older model cars from the 90s or older might use R12. Either way, you can use R134a in your car, although you may need an adapter for older cars. 

Overcharging the system and leaks are some common reasons for AC issues. However, other issues could be: loose wiring, slipping belt, low or old oil in system, and the use of wrong compressor lubricant. For a specific diagnosis, you need to check both the AC low and high pressure side readings.

Check it Yourself With a Pressure Gauge (Amazon Link)

  • Before starting make sure you know what the normal pressure for you AC system is when the car is off and when the AC is on full blast. 
  • When checking pressure using pressure gauges, you might notice the AC pressure for both the high and low pressure sides are over 150 psi when the engine is off. If so, you may have overcharged your system. You may need to release pressure from the system. Check your car manual for the correct amount of pressure needed.
  • If your pressure is low or zero, you know you have little to no refrigerant in the system and need to check for possible leaks. 
  • If you turn your car on and turn the AC to high and check the AC pressure lines, the low pressure side should read around 30 psi and the high pressure side should read around 250psi. Each car is different. Check you car manual for specific guidelines. The pressure will cycle between the low and the high sides, so wait till it clutch engages to get the correct readings. 
  • If there's pressure on both sides but the readings are off from your manual, then it's likely a part in the AC system isn't functioning properly. The compressor, clutch, and expansion valve are common problems when readings are off.    
  • If you need to add refrigerant, because you have low pressure when the engine is off. First, make sure the other parts of the AC system are operating correctly. Then get refrigerant and fill it up to the correct specifications indicated by your car manual. (Amazon Link)
  • If you need to release pressure from the AC system, it's a good idea to go to a mechanic. There are legal (Environmental Protection Agency) and safety concerns concerns if you were to do it yourself.  
  • There are other issues you can solve by yourself with more tools. So decide how involved your willing and shop around. 

Air Flow, Compressor, & Thermostat Switch Issues

I believe the best solution is to do a thorough inspection of your AC system before undertaking any repairs. This could save you time and money.

In my opinion it's better to take your time at the beginning to understand all the steps needed to undertake a project before making a costly mistake.

For AC system issues consider the following options of what could be the problem. 

1. If your condenser has blockage or restriction of air flow, the high pressure side reading might be higher than normal while the low side readings may drop over time. To rectify this problem, you should remove and blockage and clear any debris.

After removing the debris, check the readings again. If the issue persists, it is highly likely that something in your system needs repairs.


Photo Credit: http://forums.aaca.org/topic/81068-can-somebody-post-a-picture-of-the-airconditioner-on-l-8/

2. It is very uncommon to get high pressure on the low pressure side when the high pressure side has normal readings. If you are experiencing this, this might be because your compressor is not cycling in the correct manner.

The cycles are cutting in and out too rapidly, giving those high pressure readings on the low pressure side. Ask a mechanic to diagnose the problem, unless you know the compressor needs to be replaced. (Amazon Link)

3. The high pressure on low pressure side could be (with normal high pressure side readings) the result of thermostat switch malfunction. The thermostat might have the wrong range of temperature. To get normal readings, you would need to get the thermostat switch replaced by a local mechanic.

In addition to these issues remember that adding excess refrigerant to an AC system can cause problems with the AC temperature, the condenser, and other components.


If your AC system is not working properly, it's time to do an inspection. You can diagnose many problems yourself by using a pressure gauge that shows the high and low pressure line readings. In addition, you can check for leaks yourself doing a visual inspection, using a kit, or an electronic sniffer.  

But I advise that you contact an experienced mechanic for the best results when doing bigger repairs and releasing pressure from your AC system. Mechanics have all the right tools, and experience to diagnosis and repair your car's AC system. 

Additional Questions

What should I do if the condenser fan doesn't turn on? 

With the engine on, turn the AC to high and open the hood. Wait 5 minutes or til the car reaches normal operating temperature to see if the condenser/radiator fan turns on. If the fan doesn't turn on, turn off the car and wait 30 minutes for things to cool down.

Check to see if anything is blocking the fan from turning. Check electrical connections. If all seems fine, you may need to replace the fan, the temperature sensor, or something with the circuity, like the fuse for the condenser fan.