Mazda 6, 2.0L > 2.3L Engine swap.
 

Hello Dear Mazda guru's.

Under an attempt of replacing my buddys 2.0L gas engine to a 2.3L gas engine we've encountered a weird problem, so here's what we've done:

We have replaced almost all electrical component: engine wiring, the fuse-box, BCM box, ECU, Light module & immobilizer.
We have replaced The Ignition switch and key.

So far so good, all that i can think of that could be different has been replaced with parts from the Donor car.
However when we try to start the engine, the lamp for the immobilizer is blinking and the starter won't turn.

Upon further inspections with a Delphi tester we found that we can't communicate with the ECU and all other parts of the system (ABS / BCM) Reports an error between the Canbus and ECU (U0100, U1900)

We've tried the old ECU, BCM and immobilizer / light module and 3 different speedometers as well.
all those things without luck.

Is there one smart guy that can think of something we haven't yet tried?

Light, power window, radio and such works so it's not a problem with missing power. Also if we short circuit the starter it turns so it's also confirmed working.

If needed we can provide, Delphi Test data & wiring diagram over both engines.

Thanks,