Originally Posted by danr1

I noticed most the sensors you list have something in common, the are all interconnected via a GRY/RED wire. Your non working self test port is included in that.

"633 - 4WD switch is closed" would be the only one not included in that.

It could be a burnt out bulb and your hung up in 4 wheel and don't know it. Try spinning the front drive shaft, it should spin if in 2 wheel and the hubs are unlocked.

If it won't spin you're driving around in 4 wheel engaged or part way engaged.