i have a 94 civic cx headers and front cat. with downpipe and o2 sensor. my friend has a 98 civic dx and he needs to pass smog. can my header/cat/downpipe/o2 work for his??

also does a ex/si o2 sensor from a ek work with a dx?

 

Depends on the smog guy. If its a state ref he might know the difference on OBD1 and OBD2 header like that. 96-00 o2 sensors are the same. And the primary is the same as 92-95 (minus HX, CX, VX)