I'm getting both codes 42 and 43. I had drivability problems some time ago, but an extensive tune-up seemed to take care of the problem. Only thing is, I still experience it once in a while. Both codes are "Electronic Spark Timing Circuit" but 43 is related to the knock sensor. I'm not sure exactly where to begin, but it appears the ESC module has been replaced sometime in the past. I don't know about the knock sensor itself. Any ideas greatly appreciated!
At one time, the ESC ckt would do a self check that would increase timing to verify that the knock sensor is working, if it did not detect a ping, it would throw a code. I was told that this was disabled eventually. But no way to know for sure.
So it might be a knock sensor fault happening. As for an EST failure, my guess would be that the EST (the line that the ECM uses to tell the EST module in the HEI) to increase or decrease the timing was not working. So it will go to bypass mode, which is a fixed timing mode, which will throw a code.
Make sure the grounds between the block and firewall and battery are in good shape, they have cause wonders of problems. If in question, I always run a new fresh ground from the device I want to test to the battery and if I can, to the ECM.
I'm actually in the process of upgrading my grounds now. It's a "do it when you're not fixing something else" project. Fleet Farm had an 8 inch 1 guage ground cable for $6.99. I managed to bend it around and get it from the battery to the body. Now I need to get from the block to the battery a little easier.
Your knock sensor on your 5.7l is located on the passenger side of the block just in front of the starter and just above where the oil pan meets the bottom of the block. I recently replaced mine but I still have a miss as if the esc is in bypass mode which retards the timing. You may have to unbolt the starter and move it to one side if the knock sensor does not want to come out peacefully, but It isn't that difficult to change if you don't mind being under the car.