91 is a F-CAN communication, ABS off, ABS system indicator OFF, VSA On, VSA activation indicator ON The FSM says to trouble shoot all other codes first.
Then for a example, if you were down to a 68 & 91
Since 68 is a Brake Pedal Position Switch and 91 is a communication issue between.
If you only have a 91 code then here are some pics of steps for you to chew on.
There are more steps, but I would prolly just clear and see if it comes back, just me though.