I wrote asking about two codes, one being the egr valve code and the other being the piston position sensor code on a 98 hx standard. I put some seafoam in the tank, just a half a can and other than that I didn't do anything to it and just a week later the engine light went away and I scanned it just to be sure and it pulled no codes. I'm expecting it to come back since it went away on it's own, but I figured I would have to clear the code for it to go away.......baffles me anyway