Pre OBDII now qualifies as antiques, which I think are emission exempt in most states. Post OBDII, codes not recently cleared is also reported by OBDII. It is

Pre OBDII now qualifies as antiques, which I think are emission exempt in most states. Post OBDII, codes not recently cleared is also reported by OBDII. It is just software, and the software can pretend what the programmer wants. So I maintain my answer: always