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Came across a few of these at BMW, ironically they came in for electrical faults under warranty and communication bus faults with the instrument cluster, as soon as I pulled the cluster to check the wiring I found the small modules fitted, they basically take the speed/distance input signal and manipulate it to a lower reading.
Disconnected it to fix the car and they rang him to say his warranty was now void on certain items and in one of the cases the lease company took the vehicle back :D
Mileage is often stored in a few different control units these days and can throw up manipulation faults as that speed signal is used in so many things engine/gearbox management etc
Fixedwheelnut
Oh, I forgot to mention this - when I was getting the C55 serviced the service manager mentioned that the owner of the local fish and chip shop had an M6, on a lease, and that the "first thing he did was fit a mileage blocker, got it on the lowest mileage lease he could so it was cheap and will make sure the car only registers what the lease says he can do per year", which adds a new and terrifying dimension to car buying.