Hi.
I just wonder if the major problem here is the training given to these so called "Mechanics" is up to the mark. From what I can see is they are reading the codes and then have no idea how to proceed. It's all very well saying the TA is a high tech engine which it is but it still follows principals that that were laid down in the 19th century.
All the way through this thread I have read about the "garage" stabbing about in the dark with absolutely no fault finding. I have been in Electronics all my life and not only that worked in a garage workshop for many years at weekends stripping engines, gearboxes, clutches etc Oh and yes rebuilding them. The "mechanic" should be able to read the codes and work out what they mean and then proceed. The battery has been mentioned, It's a good staring point, earth's should have been tested, the loom should have been examined for chafing, plugs and sockets checked for loose connections or pins, time consuming yes but the adage of getting a repair done first time round seems to be sadly lacking here.
There could be an intermittent fault in the ECU or BCU or any other sensor, the senior tech should have been allocated the car and left with it till repaired, that way the customer would have been happier and Fiat wouldn't have egg on their face yet again.