Code 86 - Analog/Digital Error (3.4L "F" Body): Notes
WARNING: This page is about a different car, the 1993 Pontiac Grand Prix, 1993 Pontiac Firebird, 1993 Oldsmobile Cutlass Supreme, 1993 Chevrolet Lumina, and 1993 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.
If any other codes are present, use those charts first. The Analog/Digital (A/D) multiplexer chip is an internal part of the ECM. Some ECM sensor inputs lead to the A/D multiplexer. If any one of them shorts to battery voltage for more than 15 seconds, the A/D error diagnostic code will set, resulting in a Code 86.