my 91 DX has set off codes sayin the o2 tdc and crank position sensors are bad I want to know if i need to replace all these sensors with the motor still being stock i want to add on a cold air intake hedders and cat back will these effect the computer readings?

 

wtf are 'hedders'?

 

As long as your wiring is ok then its safe to say that those sensors you listed are needing replacement. Do yourself a favor and fix those items first before you start adding performance parts. One thing to remember, our 4-cyl cars have a header not headers. Just a FYI.