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Q: "Service Engine Soon" Warning Light on 1997 Infiniti I30

Car has 78,000 miles on it and light keeps coming on and staying on. Codes 0325 + 0400 first detected. Light reset 3 times. EGR valve and "knock" sensor have been replaced per repair shop advice (plus the starter), and the light has come on again after all repairs (after 9-10 engine starts). No performance problems (still gets 21 MPG in town) What to do next?

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A P0400 is not an EGR valve code, it is a problem with the EGR system, usually too little or too much flow. It is VERY rarely the EGR Valve. The knocking is being caused by the EGR system problem The Knock sensor did not have to be replaced, it was telling the truth. You need to find a sharp shop as far as emissions diagnosis goes. I would not go back the shop you have been using. They don't understand the systems involved or how to really trace the cause of codes. I can tell this by repair actions. The knock sensor code will usually follow an EGR code, if the EGR flow is incorrect. It is best to first repair the cause of the EGR code. Many times, once the car has proper EGR function, the knock sensor stops setting codes.
I have a 1999 and my check engine light has been on and it came up as the EGR valve and it sometimes jumps when idoling. i'm not sure how to approch it because i dont thing it is the actrual EGR valve.
code P0325 is in all Nissan/Infiniti's so don't bother with it. THe car runs the same with it in memory or not and will not set the light. THe p0400 is the transducer below the egr valve.
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The EGR passageways frequently get "clogged"with carbon setting EGR performance codes. Check the knock sensor performance I have seen bad knock sensors on Nissans) but frequently Nissans set a knock sensor code even thought there is nothing wrong with the sensor. Thoroughly cleaning the EGR passageways may make the knock sensor code go away. Dan is absolutely correct about the EGR system performance being below standards can set a knock sensor code.
My reason for posting is I had a similar problem with a I30 and the electrical harness was bad (due to heat) it was cracked a few inches away from the knock sensor itself and as the harness flexed when driving would intermittently store a knock senor problem.
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