My a/c cuts-out on hot days when the fan is on high. Everything works great when the fan is on other speeds. Is this a problem with the ambient temp sensor or the pressure switch? Where is the ambient temp sensor located, and how can I test it? What is the purpose of the ambient temp sensor? I had the system checked this morning and the system is not leaking and has plenty of R-12. The low-pressure line develops frost over time and the technician stated the temp was too low 18-20F. He wants to replace the pressure switch for $230 (includes recycling R-12). I want to be sure this is the problem before spending the money. Thanks