Code 43 Help

Just rebuilt the engine in a 92 4.3L Astro, VIN = W. It has never had any codes set since I have had it. Now after the rebuild the "service engine soon" light is on with a code 43 (ESC Failure). It starts, runs, idles and advances great. When I rebuilt the engine I also replaced the knock sensor and O2 sensor with OEM parts. I checked the pick up coil to make sure it was okay and have checked for bad grounds. I am at a loss on where to go from here. If anyone has any pointers please help. Thanks