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Posted this on ClubXterra, but not getting anything yet so I figured I would post here as well since the engines are essentially the same.

2003 Xterra XE 3.3
So a week ago, after taking an 80 mile drive on an xterra that I just picked up a few weeks back, I got the P0400 error code. I did some research and got familiar with the EGR system and did quite a bit of troubleshooting.
I noticed that the vacuum hose connecting to the EGR solenoid was disconnected so I was like "cool, easy fix". I also replaced all the EGR vacuum lines. I drove it around for a few days and the SES light never went away and was still idling a little rough once the engine warmed up.

The problem from troubleshooting this weekend, seemed to be that the EGR solenoid was never letting the vacuum get thru to the EGR valve.
My understanding is that the solenoid always has a positive power connection with the car on, and then it gets activated when the ECU sends it a ground signal.
I tested the solenoid itself and it was activating with power applied and also had a good seal through both ports when the plunger was on and off.
I never seemed to be able to get vacuum from the solenoid connection to EGR valve.
I even hooked up a test light to the power and ground connections on the solenoid harness and it never lit up while revving the engine up to ~3000RPM.
Keep in mind, I also did the testing while the rear was raised so the engine was under load.

Today, I pretty much had enough of the troubleshooting and was like let me connect everything up and reset the ECU and see what happens. I did that and obviously the SES light went away. I drove around for quite some time and the SES didn't come back and the car seemed to run really well. I know I'm not out of the clear just yet.
My main question is, is it possible the the ECU shut down communication with the solenoid because of the SES light or does anyone else have any idea with this MIGHT have fixed the problem--if it is fixed. I was almost positive that it was the vacuum line to the solenoid that initially caused the issue as it should have.

Thanks,
Mike
 

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As someone who was a Master Technician with Nissan and spent 16 years with them, sometimes you just fix things without never knowing what you did. It's the kind of things that drive you crazy because, as you're speculating, you don't know whether it's fixed or not. Imagine working on a customer's car in a dealership and trying to decide whether you should release the vehicle back to the customer yet or not and what you are going to explain to them! Sometimes the fix is simply unplugging and reconnecting an electrical harness connector, fixing a poor contact between a pin and a terminal. Here's some things I can tell you... EGR codes can be some of the hardest to trigger, especially if the weather is cold. Also, sometime electric vacuum solenoids can intermittently stick, making it even more difficult to pinpoint a problem. A lot of EGR problems on these vehicles occur due to carbon buildup inside the intake manifold. Also, now that many of these vehicles are getting in the 15+ years in age, the rubber diaphragms in the EGR-BPT valve are starting to deteriorate due to dry rot, which can cause EGR operation problems. Since it is your vehicle, I would just wait and see what happens for the time being.
 
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