Engine won't turn over
 

I rear ended someone in regards to someone slamming on their breaks in freeway traffic.

My buddy and I have taken apart the engine compartment trying to figure out why the engine will not turn over. Starter will not engage. Tested starter at Auto Zone and starter worked but the technician said it sounded weak.
We reinstalled starter and it wouldn't engage, had weak battery problems it seemed even with jumper cables. Not sure if that has anything to do with this situation.

Could it be the ECU or something we're missing?

Bump

Yes it could be the ECU, could be that some wires got jacked in the wreck and shorted out and messed it up.

I would star by putting 12+ volts direct to the starter and see if that will turn it over, make sure first that you have a good ground connection to it and don't make the mistake of putting the 12+ to the ground connection on the starter.

If it doesn't turn over and you have good power to it, try wacking the starter with something pretty hard and try again.
If still nothing then replace it.

No sure if you know how to do this...

Get the car jacked up on jack stands, find your starter and make sure you figure out which wire going to the starter is the ground and which one is the 12+ positive wire and then make sure your starter is connected good to the ground and take a jumper cable from your positive and touch it to your 12+ post on your starter and it should engage and turn the engin over, if it doesn't and you for sure have good power to it then you need to replace your starter.

P.S. It will spark so be careful!

Thanks for the tip.. we had the starter tested off the vehicle and it worked fine but on the car it wont engage. I'll try what you suggested though.

I ended up buying a new battery and starter, still won't turn over.