What Causes Frosting on the Low-Pressure Pipe of a Car's Air Conditioning System?
3 Answers
Car air conditioning low-pressure pipe frosting is caused by a malfunctioning expansion valve. Expansion valve malfunction: If the low-pressure pipe of a car's air conditioning system freezes, it is most likely due to a faulty expansion valve that fails to sense the real-time temperature of the low-pressure pipe. In such cases, if the temperature is too low, the low-pressure pipe will inevitably freeze (the expansion valve's opening cannot be adjusted downward). This issue generally cannot be repaired and requires replacing the expansion valve with a new one. Here is additional information: Evaporator temperature sensor failure: It could also be due to a failed evaporator temperature sensor. This component is responsible for detecting the surface temperature of the evaporator and transmitting the data to the automatic air conditioning control unit. When the temperature drops below 2°C, the air conditioning system stops operating to prevent frost or ice formation. If the evaporator temperature sensor fails, it cannot detect the corresponding temperature data, leading to freezing of the low-pressure pipe. In this case, replacing the sensor is necessary.
With over a decade of car repair experience, I've found that low-pressure line frosting is usually caused by these common issues. The most frequent culprit is insufficient refrigerant—when system pressure drops, the pipe temperature plummets below freezing. Another common problem is a stuck or malfunctioning expansion valve, where liquid refrigerant rushes into the low-pressure line and instantly frosts over. A failed dryer bottle is particularly troublesome—when its moisture-absorbing pellets become saturated, water freezes and completely blocks the pipeline. Recently, I encountered an extreme case with an older car whose cabin air filter hadn't been changed for three years—the evaporator coil was completely covered in dust, preventing cold air dissipation and freezing the pipes solid. Finally, a word of caution: if compressor wear causes excessive refrigerant circulation, it can also frost the low-pressure line—this requires immediate attention as it may necessitate compressor replacement.
My car had the same issue last year, and after troubleshooting myself, I found it was the expansion valve acting up. This component is supposed to control the refrigerant flow. If the valve core gets stuck or the temperature sensing bulb fails, liquid refrigerant can directly enter the low-pressure pipe without going through the evaporator. When sub-zero liquid meets room-temperature metal pipes, frost is inevitable. Here's another tip: if frost forms particularly quickly when using the air conditioner in recirculation mode, it's likely due to a dirty evaporator surface or insufficient fan speed, causing cold air to accumulate and not dissipate. Lastly, a little-known fact: an overly tight compressor belt can drastically increase bearing load, indirectly causing abnormal cooling on the low-pressure side. I've seen three cases of this.
