By law, if your system has been converted to run 134A, a sticker should be on the compressor or displayed prominently in the engine bay. Classic Auto Air uses this machine to detect the type of refrigerant in the system. Ours was pure R12. They have seen cars with grab bags of refrigerants. One car had Propane, R12, and 134. This situation could be bad news for a shop during recovery since any of those three will contaminate a tank of refrigerant. With R12 running at $29 for a 12-ounce can, the results could be hazardous.