yeah, for newer (obdII) cars, in cali, you can't legally replace a factory cat with anything but another factory cat. but the cop might not know that a replacement high flow cat you have is not original equipment, he/she may just be looking for any cat. at any rate, it wouldn't back-fire or smell like a catless car would, and the cop probably woudn't even pull you over. When smog check time comes, i would replace the stock parts, even though a high-flow cat would probably pass. remember, a visual inspection is part of the smog check too.
