What causes high low pressure and low high pressure in car AC with no cooling?

[DeOlivia]

The reasons for low high pressure and high low pressure in car AC are as follows: When pressure gauges show this phenomenon and there's sufficient refrigerant in the pipeline, yet the AC fails to cool, the issue likely lies with the AC compressor. There may be internal leakage in the compressor, and replacing it usually solves the problem. Below are relevant details: 1. Components of car AC: The car air conditioning system mainly consists of compressor, electromagnetic clutch, condenser, evaporator, expansion valve, receiver drier, hoses, condenser fan, vacuum solenoid, idle speed controller, and control system. 2. Classification of car AC: The car AC system is divided into high-pressure and low-pressure pipelines: (1) The high-pressure side includes compressor output side, high-pressure pipelines, condenser, receiver drier, and liquid pipelines; (2) The low-pressure side includes evaporator, accumulator, return pipelines, compressor input side, and compressor oil sump. 3. Components of car AC refrigeration system: The car AC refrigeration system consists of compressor, condenser, receiver drier, expansion valve, evaporator, and blower. These components are connected by copper pipes (or aluminum pipes) and high-pressure rubber hoses to form a sealed system. During operation, refrigerant circulates through this closed system in different states, with each cycle divided into four basic processes: compression, heat dissipation, throttling, and heat absorption. These processes repeat continuously to achieve the purpose of lowering the air temperature around the evaporator.

[StLuke]

Last time I encountered this situation at the factory, I explained it to the owner in detail. Abnormal low-pressure and high-pressure readings indicate an imbalance in the refrigeration system. Common issues include damaged compressor valve plates, which prevent the refrigerant from being compressed—just like a bicycle pump leaking air and failing to build pressure. Another possibility is the expansion valve getting stuck in the fully open position, allowing refrigerant to rush directly into the low-pressure line without proper compression. Belt slippage can also cause the compressor to spin without doing any work. Additionally, it's necessary to check if the refrigerant level is too low or if air and moisture have been mixed in. Never ignore such a malfunction—shut off the engine immediately and seek professional inspection and repair. Otherwise, prolonged operation could lead to the compressor being completely ruined.

[IanAnn]

I've had this exact issue with my own car before—the AC vents were blowing nothing but hot air, and the temperature gauge on the dashboard didn't budge at all. The mechanic later diagnosed it as a failed compressor, where internal wear caused pressure leakage, eliminating the high-low pressure differential. Alternatively, it could be a faulty expansion valve that fails to close when it should, allowing refrigerant to flood the low-pressure side without being compressed. A loose belt can also trigger similar symptoms. My advice? Head straight to the shop for a pressure test and compressor check—DIY tinkering risks refrigerant leaks. Pro tip: Avoid prolonged AC use when parked—it’s brutal on the compressor.

[AlyssaMarie]

Air conditioning pressure imbalance is mostly caused by aging of core components, such as excessive wear gap of compressor pistons or stuck expansion valve springs. Another possibility is improper blockage position of the throttle device, causing excessive refrigerant accumulation in the low-pressure section. In such cases, turn off the air conditioning first to avoid damaging other components, then check minor issues like belt tension and clutch gap. Finally, professional instruments must be used to measure the system pressure curve to accurately locate the fault point. Some used car dealers may temporarily add refrigerant to cover up air conditioning problems, so be very cautious.

[Jerry]

I've handled dozens of such cases, and the root cause is always a compromised refrigeration cycle. The most common failure point is the compressor as the power source - cylinder scoring or valve plate deformation can cause a drastic drop in compression efficiency. Abnormal expansion valve opening is another critical factor, similar to a faucet that won't shut tightly. Additionally, air intrusion into the system creates false high-pressure readings on the low side. First, check if the belt is worn or slipping - that's the cheapest fix. Insufficient refrigerant typically shows low pressure, but here the high pressure won't rise, indicating the issue lies in the compression stage.

[AlexandraLynn]

From a thermodynamic perspective, low high pressure and high low pressure indicate the compressor failed to perform its compression function. Common causes include mechanical wear leading to compression chamber leakage, or the expansion device losing its throttling effect. I've personally witnessed cases where electromagnetic clutch ablation caused slippage and idle running - the tachometer showed normal readings but the compressor wasn't actually delivering power. Another hidden issue is deteriorated refrigeration oil forming oil blockages, which obstruct refrigerant flow and cause abnormal pressures. During repairs, pay attention to thoroughly cleaning and evacuating the entire system. Simply replacing components without cleaning the pipelines will lead to new compressor failure within six months.