Only on older cars, which do not have OBDII. OBDI and before that require a sniffer. OBDII has enough sensors that the sniffer isnt needed. Am in CA, and only m

Only on older cars, which do not have OBDII. OBDI and before that require a sniffer. OBDII has enough sensors that the sniffer isnt needed. Am in CA, and only my foxbody ever got a sniff test. All of my other (newer) cars, the sniffer wasnt used