About a few weeks ago I went to start my Discovery and noticed the door locks didn unlock. I tried to start it and when i turn the key there is nothing. I thought it was the battery. So i bought a new battery put it in and still no power throughout. No lights nothing. I took it to a garage and they insisted that it is the ECU. My question is i have work on various models and the ecm is seperate from the rest of the controls like the lights and doors even dash lights. Could the ECU cause this? I dont have any power throughout the entire vehicle.

 

No the ECU will not cause this, all that controls is the engine.
I would say that it is a bad battery cable, either it is corroded beyond all usefullness or it has come unhooked from the starter or where it grounds.
All the trucks power travels through the battery cables before it goes anywhere else.

 

There are also some big 100amp and 50 amp main fuses or fusible links in the under hood fuse box.

 

This happened to me recently, turned out that it was my alternator, although when I jumped mine, it would start right up.