Does your glow plug dashboard lamp come on briefly?
Is the engine ECU visible with your OBDii scan tool?
If not, then your engine ECU is probably not getting power.
The problem might be as simple as relay R7 under the dash. You can swap it with another relay to check. The MB tech suspected my whole fuse block needed replacement, but swapping the relay fixed my problem and I was on my way. According to him, fuse blocks or the wiring can also fail with similar symptoms. One can also assume ECUs can become intermittent, but I've heard this is rare.
If you do have a glow plug lamp showing, then look elsewhere. It could be the problematic "Y cable". There are plenty of posts on fixing that.
Swapping my R7 relay fixed my rig for about a year, but recently it happened once again. The defective relay had a deep pit in the middle of the contacts and at first it was just intermittent (requiring one or two extra key tries) and then it failed a couple times for about half an hour.
Jiggling R7 sometimes let my engine start, so maybe my fuse block is part of the problem.
The good news is the relay is about $15 on Amazon or ebay, listed under part number: Hella 0025422619. They may be different for other years or models; mine is a 2011 NCV3, so check yours first.
If my suspicion is right, my relay pitted because something is different with the new ECU installed during the AEM. That would explain why the second relay is failing. I need to look at the inrush current with an oscilloscope to see if that is true.
A SSR (solid state relay) might be better if there's enough inrush current to destroy the mechanical relays.
