Faulty Control Unit: Causes & Solutions

Hello everyone,
I'm having a problem with my Golf 4 ALH 66kW, manufactured in 1999 (model year 2000). According to the VW dealer, my original engine control unit, with the VW part number 038906012 M (Bosch part number 0281001979), is defective, and this was determined using a portable diagnostic device. I then purchased a new engine control unit (which was quite expensive). However, there was a change in the part number, so now it's VW part number 038906012 GK (Bosch part number 281010864). Now, I suspect that the original engine control unit may not have been defective after all. My problem is that I want to reinstall the old engine control unit, but I can't get the immobilizer adapted. I have a diagnostic connector with a laptop and VAG-Com available. I'm not entirely sure if I absolutely need the 7-digit PIN code for the adaptation, or if the 5-digit code is sufficient. But which one is it? I don't want to ask my dealer about this, for obvious reasons. I hope someone can help me.

Joda999