First check the oil level of the transmission!
C= Chassis (Transmission,ABS,....)
0= All manufactors uses same code
4= Emmision control system
0= First Number of part
0= Second Number of part
If Oil level is korrekt, a transmission failure is detected, please go to the nearest dealer to check your transmission
It may be a completly fault or only a little thing to do.
Sorry, the C0400 is not listet in my carlist. Perhaps another LKW-Dealer can Help with this code, because its the same on GMC,VOLVO,RENAULT, and so on.
greetings to Russia from Austria