Quote Originally Posted by lionelhutz View Post
You can't put power on the flash chip to program it without that power also powering the rest of the circuit assuming it all shares a common power supply.
Except that this isn't necessarily the case. For example, I have an ICP header on one of my prototype boards that goes to the pins on my microcontroller required to program it were it to go faulty. Those pins are also used to talk over I2C to a DAC, and for other things. But if I'm not powering the board from the input rail and am instead connecting to the ICP header, then the rest of the circuit is unaffected as it's not powered up. Sure the signals I'm sending to the microcontroller are going to also hit the DAC, but the DAC isn't receiving any power so it won't matter.

Similarly, if GM designed a header to allow in-circuit programming, why would you assume that programming the flash chip via this header would somehow power up the whole PCM? Similarly, if one were to make a jumper connector to flash the chip directly, why would the rest of what the chip's connected to matter since there's no power to the PCM?