Have no clue what I'm doing
 

Have a 2001 GMC Sierra 1500 vortec 5.3 engine stalled wouldn't crank back up, now cranks occasionally runs for a bit , rough like misfiring, I am getting the p0405 code, egr is brand new. Any thoughts pls

It could be these issues

Short to ground in EGR signal or Reference circuits

Short to voltage in EGR ground or signal circuits

Bad EGR valve


Bad PCM wiring issues due to chafing or loose terminals



I'd look for any damaged wires. Might have fuel pressure checked and at the age always a good idea to clean the MAF too.

Ty for the info

I forgot to mention little to no oil pressure on gauge, round the same time

Any water in the oil? Any bubbles coming out of the cooling system. Anti-freeze will tear up bearings leading to low oil pressures.
Need to run a compression test on all of the cylinders. Might be a bad head gasket if your rig is loosing coolant.