Nissan Repair: oxygen sensors, fuel pressure regulator, oxygen sensors

Question
I have a 2001 Frontier 2.4L with a scanner reading of P0420. I've taken your advice and I'm making the different checks to solve the problem, what is really confusing me is, in both my Haynes manual and my OEM factory repair manual, they show the downstream oxygen sensor after the catalytic converter. I just was under the truck again, and mine is about 3" BEFORE THE CONVERTER, can you explain this to me? Also I'm told that I have a California emmission system, can I tell from my VIN? Thanks Rich

Answer
Rich,

If you bought the car in California it is a California emmissions system.  Since the advent of OBDII there is a downstream oxygen sensor after the converter and this rats you out if you remove the converter and tells you if the converter cannot get the emmissions under control.  I would be checking up stream like the fuel pressure regulator or a stuck injector.  If the converter is messed up it is due to too much fuel going through the system and that makes it overheat and melt inside.  Or, if the car is burning oil it could kill the converter too.  So, check the fuel system for a regulator or a stuck injector.