I just did an Ap2 engine swap into my 2000 S2000 and the CEL come on with P0133 which is the primary o2 sensor. I change that with a new O2 then CEL come back on but with P0141 which is the secondary one I change that too. Then CEL come back on with P0171, P0133, P0420, P0170 four code now. I look it up and one was the CAT being bad and I happen to have an other one laying a round and change that. Then I put the 2 old O2 back in to see what happens. The CEL come back on now with P0133 again with no other codes. Took it to a smog guy to get an diagnostic on the both O2 and he says there reading fine and working right. Can a 68mm bore out throttle body be causing the primary o2 sensor to be reading wrong???