What is the reason for high low pressure and normal high pressure in car air conditioning?

[VonCamille]

Car air conditioning with high low pressure and normal high pressure is caused by: malfunction or damage of the air conditioning expansion valve. The solution for high low pressure and normal high pressure in car air conditioning is: remove the air conditioning expansion valve and check whether its working condition is normal. If it is indeed found to be faulty or damaged, simply replace it. Car air conditioning, short for car air conditioning device, is used to adjust and control the temperature, humidity, air cleanliness, and air flow inside the car compartment to an optimal state. Its functions are: 1. Provide a comfortable riding environment for passengers and reduce travel fatigue; 2. Create good working conditions for the driver and ensure driving safety.

[VonKaylee]

I often encounter several situations when repairing cars. If the internal valve seals of the compressor are not tight, the compressed refrigerant can easily leak back, and the low-pressure side won't drop. Alternatively, the expansion valve might be stuck in the open position and not close properly, or it could be completely blocked, causing excessive refrigerant flow. Just recently, I dealt with this issue on an old Jetta—the owner had overcharged the refrigerant, and when I hooked up the gauge, the low pressure shot up to 5 kg. Remember, if the low-pressure line feels colder than usual or even frosts over, it's a sign. Overcharging the refrigerant or introducing air into the system will definitely mess up the pressure. If the compressor is truly failing, the pulley may spin, but the internals won't work, and the air from the vents will be lukewarm. Always test thoroughly before taking action—don't replace parts blindly.

[VanDaniel]

I've personally encountered this issue with my own car. At the time, turning on the AC made no noticeable difference. Diagnosis showed the low pressure was abnormally high, hovering around 3.5 kg/cm², while the high pressure remained steady at 15 kg/cm². The veteran mechanic immediately identified a stuck expansion valve, and upon disassembly, we indeed found a small metal shard jamming the valve needle. Another time, slight deformation of the compressor valve plate caused poor sealing - this kind of wear develops gradually, initially manifesting as slowly rising low pressure. If this occurs right after refrigerant recharge, it's likely either overcharged or contaminated with air due to incomplete vacuuming. Also, never overlook the cabin air filter - severe clogging reducing airflow can cause evaporator icing, which in turn affects system pressure. For more accurate diagnosis, I recommend simultaneously measuring the temperatures of both AC high and low pressure lines.

[DiHeidi]

In principle, low pressure being high while high pressure remains normal indicates a decline in compressor efficiency. Worn piston rings or intake valve plates can lead to insufficient refrigerant compression, causing some gas to flow back during the suction stroke. If the expansion valve fails to regulate properly, liquid refrigerant can directly rush into the low-pressure line, causing liquid slugging. There have been cases where residual moisture after repairs froze and blocked the expansion valve screen. Mixing refrigerant types or incorrect proportions can also cause this—adding R12 to R134a refrigerant will create chaos. Here’s a little extra knowledge: when the front condenser is too dirty and heat dissipation is poor, high pressure will spike while low pressure remains unchanged, which is different from the situation you described. If you’re really unsure, it’s recommended to connect a manifold pressure gauge and test it—idling with the AC on for ten minutes is the most accurate.