OK, I've just flashed the newly un-bricked LS PCM to the stock SS program, except VATS set to 0 instead of the default of 1. Without power to C1-24, it cranks, fires one cylinder, and then just endless cranking. I can get another single-cylinder fire only by turning the key to off before cranking again. With power to C1-24, I get a DTC as soon as I turn the key to ON, P0452. This is the fuel tank pressure sensor, which my car does not have, so that code is expected. That code being set only when I hook up C1-24 to battery power leads me to believe that it is required to power on this PCM. Although having said that, my bench harness does *not* have power to that pin and I'm still able to connect to it with my OBDLink SX and read/write with WinFlash. So is it required or not?! And now, when cranking it starts up, runs for a few seconds and shuts off. Still sounds like VATS... I also tried hard wiring the fuel pump and have the same result, does this eliminate VATS as the cause or does the 04 Impala do more than cut fuel pump power? I also get two more DTC codes set after that, P0036 and P0449. P0449 is another EVAP code, which is expected. P0036 is an oxygen sensor code, and will require further testing some time in the future. But none of these codes give me any clues regarding the current issue.

As a side note, unrelated to anything else here, I've found that when I change the VIN with WinFlash, the PCM reverts itself back to its original VIN after a power cycle. The VIN doesn't matter for my application, I just thought it was a noteworthy observation.