FULL RESET FORZA 5
(ie. remove your player data like carreer, cars, etc, and start from scratch)
When you see the Splash Screen (the screen with the orange McLaren P1 and text that says “Press A to Begin”), do the following:
Hold down both shoulder buttons (LB + RB) while holding both triggers (LT + RT) and then press the Y (yellow) button at the same time.
If done correctly, you should see a confirmation pop-up confirming that you want to reset your profile. Choose “Yes” from the menu and then press the A button to reset your profile. A successful profile reset should unblock you from proceeding through the game.
WARNING: This profile reset will permanently reset your profile including: career progress, tokens, credits, liveries, etc.
Only do this if you are absolutely sure you want to start over.
SPLIT SCREEN PLAY
Turn on Xbox log in to my account.
Select Forza and start the game.
Try to get to the main menu and Choose free play to get to split screen.
Press Xbox button on second controller log in as guest "my account(1)."
Press menu to activate in Forza.
Go to select car with second controller and I only have the standard rental cars to choose from.
First player has all the cars from my garage.
Some (or most?) users run into the issue that to add the second player it says to press menu (start) button to start. Whenever you press the menu button on either controller it just takes me back to the main menu. This is most likely related to not having a second player assigned to the second controller. Make sure you have the users signed in on separate controllers. Chances are both of you are on controller one.
SWITCHING GAMES TO RESTART
Some situation require a reboot of your XBox One, I guess Microsoft forgot to implement the CTRL+ALT+DEL button, but in most situation you really don't have to reboot. Another way to completely restart a game is to load another and then back to Forza. For example: "Xbox go to FIFA 14" ... "Xbox go to Forza Motorsport 5".