Air temperature not cold enough
 

2003 Montero Ltd - 3.85 liter engine. The temperature of the AC is not cold enough even when AC temp control set at lowest setting. Problem worsens as outside temperature increases. Have not taken vehicle to dealer - quite a drive as most dealers in my area no longer sell/service Mitsubishi vehicles. My mechanic was not able to diagnose from computer codes. Compressor runs, refrigerant charge checks out OK. Could it be a bad expansion valve?