I bought a Go Plus Pit Stop (the set) used. The CU did not work well, one track dead on arrival. I bought a new CU. It worked fine. I installed the app and then the cars could not be started together. I had to let the ghost car get 6 ft (2m) away before starting the other car. I sent it back to the dealer. The new one did the exact same thing. I got another warranty new one from Carrera yesterday. Figured I wouldn't install the APP. The CU started out doing the short thing as soon as I tried it. On a whim, I put on my two other controllers. Works perfect. Tried every combination of controllers, all good! 
Tried another bad CU, changing controllers did not fix it. Even stranger, the new one went back to bad as soon as switched back. I switched controllers, (I didn't keep track of which was which), and it fixed it again! Again, any two of the four controllers and it keeps working. I'm at a loss, but at least it is working! I'll use a mechanical lap counter and never install the APP.
If yours shows a short, flashing blue light and buzzing while cars don't move, try letting one get 2m ahead. Worked on all three of mine when starting together would not work. You might try switching both controllers with a friend, if it fixes the CU you can put yours back on.
