2012 Suburban, 5.3, 4x4, LTZ
No crank no start after the dash displayed stability control error, traction control light came on, engine reduced power, christmas tree version...
I was driving down the road, no issues, no service light, but the air bag light has been on since date of purchase, then the above errors pop up and the engine dies while driving.
I thought this was going to be the alternator went out kind of situation but the battery was ok and the new battery had no effect.
No crank and no start condition ever since.
Ive checked all fuses in the cab and engine bay.
Ive checked all the relays that i felt were relevant. Including start / run, crank, power, ect. No fuel pump relay or fuse, it has the FSCM
I can bypass the starter relay and the starter works but the engine doesnt run with the key on.
Ive noticed that the fuel pump runs the entire time the key is on, idk if this is normal.

Ive attempted to test the FSCM, and it seems ok but im not completely confident
Ive tested the ignition and it seems ok., but not confident
Ive checked most if not all grounds.
I dont have a good code reader but a friend has a cheapo and no codes are present, bc I made a huge mistake and left the battery off all night.
I found that the voltage was 11.59 at the bcm on x4 pin 21, which is supposed to be the crank / run trigger. This seems problematic to me.
Ive removed the engine bay upper fuse assembly and checked for corrosion on the main 12volt supply wire.
Ive checked fuel pressure at the main rail at the engine, fuel pump running, engine not running, and had 58 psi.

How can I more systematically approach this to troubleshoot?

What would interrupt the ignition circuit?

Does anyone have any input and possibly a way to test the wire / circuit that is the ignition trigger to the starter relay from the bcm?

I'm open to testing / retesting any and everything.

 

Does anyone know how to test the ignition switch with a test light and or voltmeter on a 2012 suburban?

Ive got the 5 main wires to the switch.
It has the following wires.
White - has 5 volts when no key, and stays around 4 or 5 in all positions
Pink Black- Has 12 volt when the key is inserted
Red White - Battery Pos 12 volt
Brown - 12 vdc when key in 1st pos or acc
Pink - 12vdc when in 2nd pos / run
This is a summary of what ive found.