Im nearly convinced only because I dont have any confirmation from the Heavyweights. I realize it may be trivial, I feel this is crucial for better understanding of donor cars in events and being better able to recreate events in Arcade TT for those of us who dont go online but would like to recreate the events anyways.
At 200 miles the 9C91 code appears on my cars, just as it is on donor cars.
The cars in GT used to take around 150 to 200 miles to break in.
My theory is this code is used to make the car at fully broken in with clean oil power. I believe that was more in play before when cars needed a break in period, but has not changed since break in is no longer required, the code still gives fresh oil power.
I dont see any other reason a code that only appears on cars that have the required miles to break in the car would be put on never driven donor cars.