Well I replaced my 98 motor a few motors ago with a 96 lesser miles motor. Its been running smoothly and everything but I noticed that when I put 98 ecu in there is no check engine light but when I run my TS 96 ecu, it trips the CEL, why is that? I pull the code and it said rear o2 and fuel control module... If the 96 didnt have a rear o2 why would the 96 pull an rear o2 code?

 

Why do you say that a 96 has no rear O2 sensor?
My 96 FSM shows a 3 wire titania type 02 sensor for the rear.
According to the FSM, the fuel pump control module FPCM (code P1220) is for Cali-spec only.
All the sensors in the 96 and 98 are the same with the exception of the type of rear O2 sensor used.

 

96 has rear O2 its in the cat.....

 

the motor doesnt make any difference, you still have the same O2 setup.

 

the 02 setup is the same, but are they interpreted the same way by the ecu?

maybe the 95-96 ecu's have a different numerical value than the 97+ which have the other o2 in anothe location.

 

The 96 does have a rear o2 sensor that is not the same as your 98, that is why you get the code.

If your confused about why a 96 motor and a 96 ECU are giving you a CEL, remeber that all the wiring and sensors are still 98.