I think I just answered my own question. 2344051 says upstream left while 2344150 is upstream right. Uh, which is left? looking at the front of the vehicle or back?I just received two Denso PN#'s 234-4150 and 234-4051 O2 sensors. Is one designed for the inside pipe vs the outside pipe? If it doesn't matter, why the two different part numbers?