I posted this on the Astro forum but didn't get any help. Thought I would give it a try over here.
Just rebuilt the engine in a 92 4.3L (C.P.I.) 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

I thought a 43 was the knock sensor circuit.

According to my GM code book-code 43 is electronic spark timing (EST) circuit or electronic spark control (ESC) circuit problems.

After doing some checking...it is all the same thing.

http://www.syty.org/old/d&e-code43.html