I recently had to take my wife's 1990 N/A 300zx thru emissions and it failed. I did every thing. Changed the plugs coil packs. The injectors are brand new. There is a very small exhaust leak in the center section. I mean very very tiny. I found that the PVC valve hose was disconnected a few days before I took it thru. However I reconnected it and checked the rest of the hose connections and they were all good. I hear lots of people say that its very hard to get these N/A 300zx to pass smog.