Ok so this is a little weird...
Starting from a freshly disconnected and reconnected battery, everything is normal. Zero voltage at the ecu, except for the backup power supply, which is normal from what I gather.
Once the key is turned to on/ign, everything has power, still normal.
Car will crank but will not start.
If the key is returned to off, everything visible shuts off; gauges/accessories. However, the ECU still receives 12v, and therefore so do a bunch of sensors and the fuel pump and whatnot. The only way to kill the power at this point is to disconnect the battery.
I assume the main relay is the culprit, as it supplies power to the ECU, but... The main relay is triggered by
the ECU. The 'self shutoff control' is what energizes the main relay.
"Must be the ignition switch sticking." Well...
Without power, the switch is fine, but with power it's the same as the ECU: no power until it's turned on, and then the power just stays on no matter the keys position.
Tried a spare ECU, no change.
Sorry for the jumble, anyone got any ideas? I've been tracing the diagrams and checking voltages and nothing's sticking out except for EVERYTHING. Only thing to do is replace the main fuse, but I don't have time to take my dash out tonight.