When I say I was able to read without it, I mean that it was not connected. I believe the white plug only has the one wire that supplies power.

 

The programming pin (FWE) exposed on the diag connector takes ~16V to enter programming mode, if memory serves; there's a step-up circuit that doesn't always hit that on the older OpenPort 1.x designs when battery voltage is a bit low, which is why you see a lot of suggestions in the archives of "charge your battery" when flashing fails.

You don't need it for reading; it's only used for putting the ECU into one of the programming modes. As kaj said, it can be completely physically disconnected, and reading the ROM with EcuFlash will work just fine.

If you want to write changes to the ECU, there's quite a bit more work to do than just applying 16V to FWE; there's a fairly involved rain dance that you have to perform via serial (OBD-II) as well, which EcuFlash expects to be able to do by itself via a working OpenPort cable.

(At some point, I should probably write up the flashing protocol and put it on the wiki. I'll probably leave writing the kernel you have to upload as an exercise to the reader though. )

 

That makes sense, because if I don't keep my car on a battery tender (my car sits a lot) and go to do any reflashing, I get an error.