Have these codes been there before the modification, or have they always come up? The solenoids are not very expensive and the EGR valve is easy to clean. I have had to replace a couple of them that held vacuum. I had the muffler and catylast off mine for a short time, and the computer didn't care. The reason I put them back was because there was no increase in mileage or performance, and I preferd a quieter engine sound.
94 Chev Blazer, 6.5TD, 3.42, 4WD,Gear drive,,Remote FSD behind drivers headlight,,Mobil 1 in difs and trans,Rotella 30W, Over a quarter of a million miles !! Member -1.94 club.