The generic info on SID 081 is an exhaust gas pressure sensor, FMI 04 means low voltage. Probably a sensor or wiring issue. Since you're keeping the engine a secrete, that's the best I can do for you. I might be able to give you more info if you could at least tell me what color the engine is.
Yellow is usually a Cat, but they didn't have exhaust pressure sensors in 06. You can look for a data plate on it, or you can call Freightliner with your VIN and ask what engine it has.