What condition could cause the system not to respond to an adjustment to the expansion valve?

The system not responding to an adjustment of the expansion valve can indeed be attributed to the presence of oil in the evaporator. When oil accumulates in the evaporator, it can hinder the refrigerant’s ability to absorb heat effectively. This can lead to reduced heat transfer efficiency and may affect the performance of the expansion valve.

In a properly operating evaporator, the refrigerant should vaporize fully, absorbing heat from the surrounding area. If oil is present, it may mix with the refrigerant and negatively impact its thermodynamic properties, making it difficult for the expansion valve to function correctly. This can result in an inability to achieve the desired cooling effect, as the refrigerant is not able to evaporate as it should, regardless of adjustments made to the expansion valve.

Other conditions such as low ambient temperature or excessive refrigerant charge can affect system performance, but they do not directly interfere with the action of the expansion valve in the same way that oil in the evaporator does. High discharge pressure generally indicates a different set of system issues which are not necessarily linked to the direct functionality of the expansion valve with regard to system adjustments.