Rob Weiss wrote:
Bob, do you live in a dusty climate? One silly thing that causes P0402 EGR Stuck Open codes is the the little sponge filter on the EGR solenoid gets clogged up. When this happens, solenoid holds a vacuum even when none is called for. So you get EGR flow when none is called for. Carefully (use a pocket knife) pull the cap off the EGR solenoid and clean the sponge filter inside. You may break a couple of the tabs on the solenoid cap, but that's okay. It will still hold.
That seems like good thinking. I do, in fact, live in a dusty area. But if you read my original posting this thread, you'll see that I wrote,
?I then tested the EGR Vacuum Regulator Solenoid, which controls the EGR valve, and determined that it was malfunctioning, so I replaced it.
This seemed to cure that problem, but several weeks later, it returned, and I was again getting the P0402 code. On further testing, I determined that the EGR valve itself was failing to consistently close all the way when it should, and that it was the EGR valve itself, and not some other component, which was failing.
By way of verifying that it was the EGR valve itself, and not the solenoid, I disconnected the vacuum hose from the EGR valve, and had the P0402 code occur while the EGR valve was thus disconnected, and while there was no way that the EGR valve could be receiving a vacuum to open it. Without a vacuum, the EGR valve should have remained tightly closed, but the P0402 code would seem to indicate that it was slightly open.
