Answer to: Optimized Idle fails, check engine light.I am maintenance director at MAG Carriers/MAG Transportation. We have all 2015, 16, 17, and 18 Freightliner Cascadias. I have had this problem with a few of the dd15's. Check for loose ground wires on the starter. I had one that the nut holding the wires on the ground stud of the starter was missing/gone all together. It's a common problem with them elsewhere as well. This also could cause charging issues which could be another cause for not only the optimized idle to fail but startup all together. Only other cause I've seen for this fault is yet another very common issue for quite a few other electrical faults with the new Cascadias and that's the main power disconnect. It can be unplugged and not active in the system at all but make sure it is in the "ON" position before disconnecting the switch from directly through the floor. It is a small 3 wire plug and very easily accessible for disconnecting.