I have this really odd problem with my C5 RS6....
Sometimes after a long or spirited drive (or in hot weather) she refuses to start for about 20min to an hour after switching off. Doesn't even crank and you cannot even hear the fuel pump priming. She will however start if jumped from another car. I have replaced the battery and it has helped as it now happens less frequently...but it hasn't resolved the problem.
I was thinking heat soak on the starter solenoid maybe? But would that account for the car being able to start when jumped?
Any advice...
Oh, No codes...