CAT 3126B no start

[Aussie Steve]

You have an electrical fault for sure. Cats have two speed timing sensors, one picking up crank speed l/h side of block (it's usually a pain in the backside to get to) and one measuring cam speed at the rear L/H of the timing cover. If they loose the signal from one sensor they will swap over to the other sensor but will also log the fault. They will run on one sensor. So it might be that something was disturbed/dislodged etc when the new harness was installed. The engine has to crank above 150 rpm to fire and turn a minimum of three full revolutions before it will fire.
The ecm has to be able to see that the engine is turning fast enough, maybe reinstall the sensors as per the procedure and try it again