Why is there no pressure in the low-pressure side of a car's air conditioning system?
3 Answers
Here are the reasons for no pressure in the low-pressure side of a car's air conditioning system: Damage to the outdoor unit of the car air conditioning: If the fan of the outdoor unit is damaged, it will result in excessively high exhaust temperature or pressure; if the internal fan is damaged, it will cause frost formation, and at this point, it's necessary to replace the damaged internal or external fan. Failure of the compressor's capacitor: When both the internal and external units of the car are operating, but the compressor does not rotate, it will lead to overheating. In this case, it's necessary to check whether the compressor capacitor has poor contact or is damaged, and replace it promptly if damaged. Reversed live and neutral wires of the air conditioning power supply: This phenomenon usually occurs only during the installation of the car air conditioning, requiring the owner to reconnect the live and neutral wires.
The other day at the repair shop, we saw a car with zero pressure in the A/C low-pressure line. We checked several areas. The most straightforward issue was that the refrigerant had completely leaked out—there wasn't enough refrigerant circulating in the system, so of course, no pressure could be detected on the low-pressure side. Next, we used a UV light to inspect the lines and found fluorescent dye leakage traces at the condenser connection. When we disassembled the expansion valve, we discovered the valve core was stuck. If this gets blocked, refrigerant can't enter the evaporator at all. Another possibility was the compressor clutch not engaging—the belt was turning, but the compressor internals weren't working. Finally, a reminder: if the high-pressure line doesn't feel hot either, it's likely the entire A/C system has completely failed, and a full inspection is urgently needed. Just topping up the refrigerant is a temporary fix, not a solution.
Just dealt with a similar issue a couple of days ago—it was quite alarming to see the low-pressure gauge needle completely frozen. Initially suspected refrigerant leakage, and sure enough, spraying soapy water on the pipe connections revealed bubbling. Another possibility is a completely clogged receiver-drier; if this canister is blocked, refrigerant simply can't flow to the low-pressure side. A shattered compressor piston is even worse—the belt keeps turning, but there's zero compression of the refrigerant. For some vehicles, it's also worth checking if the pressure sensor has failed, where the system shows pressure but the computer reads no error data. The most frustrating scenario is an expansion valve stuck in the closed position, causing high pressure to build up on one end while the low-pressure side remains empty. If your A/C completely stops working, I recommend first sniffing for any traces of refrigerant oil—a strong smell usually indicates a significant leak somewhere.
