This E.C.M. fits the following vehicles: 2005-2006 4.3L GMC Sierra 1500 & 8.1L Sierra (2500HD, 3500). This is for one service number 12589462 ECM (Engine control module) that is programmed to your V.I.N. (Vehicle identification number) to exactly match your vehicle. After you purchase the E.C.M. you will need to provide us with your V.I.N so that we can program the E.C.M. E.C.M.Two things may have to be done after you install your E.C.M. First an antl-theft relearn may have to be performed. If this is the case the truck will not start. The second thing is that a crankshaft variation relearn may need to be performed once the vehicle is running. This will be evident if the check engine or service engine soon light comes on once the vehicle is running. You will need access to a scan tool capable of doing this procedure to do this and clear any related trouble codes (Code P0315). If this is not available to you a repair shop will have to perform this and clear the related trouble code. The vehicle will run normal but the ”service engine soon” light will come on and a trouble code will be stored until this is resolved. Instructions for both procedures will be included.