It is a small metal, magnetic cylinder that sits above the transmission. It has been called by different names, one of them is the speed sensor. The car has 3 times coded that something is wrong with that piece. Once the code is cleared it does not immediately come back. Volvo ordered a new one of those & what arrived did not fit. Even though it was ordered with the serial number. The car was bought in the US but now we live in Canada. The dealership swaped that piece with one from another S80 they had in the shop. There was no change. The shop thought they had it solved. They thought it was the battery cable itslelf. They replaced it, it made no difference. I think we will rent a car for our vacation starting this week & let them work on it another 10 days. I find it hard to believe that a car that runs perfectly once started may have to be abandoned because it is having problems starting. It is extremely discouraging. Sandy