The check engine light came on twice in our old PW Excel with a Ford V-8. The instances were several years apart. The light was yellow and Ford told me it was OK to drive the van. In both cases I am convinced I got a bad tank of gas. We were on Indian land in Utah/Arizona and bought gas there. As I understand it, the gasoline on Indian land does not have to comply with EPA rules. In both cases the light went out after I burned off that tank of gas. Your problem might be related to the gas (or diesel) you are using.