I'm new here to the forum and I have a question about my 98 YMCA sierra 2500 2wd with a 6.5 diesel. Every time I drive the truck I get a P0400 and P0501 code for Egr flow and Egr flow insufficient. The truck is all stock other than I cut the muffler and converter off and put flex pipe on instead. I have done a few tests and I have vacuum to the first two black solenoids on the drivers side while the truck is ideling but I'm not getting any vacuum coming out of the middle solenoid. I checked all the vacuum lines and none of them are collapsed and they all look good. Does anyone know why I keep getting these codes. They come on after 20 or 30 minutes of driving and is it possible the codes are on because I removed the converter and muffler? Any help would be greatly appreciated.