2007 Yukon will not start

Hello to all. Here is my situation.
I am working on a 2007 Yukon Limo conversion. The customers complaint was that the truck started "knocking" and then shut off. They were able to drive home. Upon inspection, we can get the truck started with starting fluid. But will not start normally. We have installed a new fuel pump and achieve 50-55 psi of fuel pressure. What is weird is that when we hook up a Snap On Modis scanner in Global OBD mode, the scanner cannot communicate with the ECM.

We have 12v at the injector wires using a test light. We forgot to bring the noid light.

Any suggestions as were to start looking for a solution?
Any and all will be greatly appreciated, the customer has the limo booked for this sat. 19th of June 2010.

Thank you in advance
Blessings
Chuck Butler Jr
Oh yeah 5.3 l

You need to check for injector pulse for starters, the modis may not communicate if the body feed fuse is out, or the pcm is just ng. Look at all the injector fuses also.

Thank you maxwedge. The customer needed it sooner than we could figure it out, so. Will post back the findings. We are leaning towards the PCM>. Limo vehicle with taps and grounds everywhere. A Royal Coach, out of business.

Blessings