2015 200 wont start
Had less than gallon diesel put in it taken out not
A gallon of diesel mixed with a full tank of gas would not be a problem. Are you trying to start it on straight diesel?
If the fuel level was very low when the gallon of diesel was added that will cause it not to start and the fuel system may have pure diesel in it. The system will likely have to be drained and a new fuel filter installed. Probably will require a local mechanic to get it straightened out.