I have tried to immerse myself more as to the differences in Audi transmission control modules a.k.a . transmission control units, specific to the Audi A6 Quattro or A8 cars of the 1997-2005 era, those fitted with the ZF 5HP-24A transmission. I don’t have any brilliant insights yet, but I’m learning. And, I like to have my data first-hand, so there I was, last night, with a stack of half a dozen different Audi transmission control modules, trying them in our two different Audi project cars.

Yes, two. We now have two 2000 Audi A6 project cars with the 4.2 engine and the ZF 5 HP-24A transmission, and the 4B0927156BS transmission control module. Both are failing in exactly the same way: in “drive” the transmission slips, and then there’s a a jarring “thump” and the car goes into “limp-home” mode, and the instrument panel indicator reflects this.

Swapping the two transmission control computers didn’t fix the problem in either car.

In each of these two cars, we have also tried two transmission control modules with part number 4B0927156CT … meaning basically the same part but a variation. On the blue car, the car immediately goes into limp-home mode, so the car treats that as very different from the 4B0927156BS part. On the silver car, the car treats the 4B0927156CT part the same as if it were the 4B0927156BS part. The car tries to go, then slips, then collapses into limp-home mode. I’m VERY surprised that the same transmission control unit is treated differently in two cars of the same model year.

As I understand the Europaparts explanation of the part numbering convention used by the VW-Audi Group, the first three characters (i.e., “4B0”) are the model for which this part was first introduced.  According to the CDIF/3 website, “4B0” is the C5 Audi A6, made from 1997 through 2005. So, okay, that transmission control unit was first made for the C5 Audi A6. But, might something else work too?

The 4th digit, “9,” means it’s electrical. The next two digits are the sub-group. “27” is presumably “control module.” The next three digits, “156,” identify this particular type of control module, probably the transmission control module for the ZF Tiptronic automatic transmission.

Lastly, the letters (“BS” or “CT”) are a variant or modification code. So visually the “BS” part is the same as the “CT” part but they are not internally identical, nor always interchangeable.

On a hunch, I visited a premium automotive junkyard, and bought half a dozen transmission control modules for the ZF Tiptronic automatic transmission. They all have the core part number of “925156” though they are different as to the first three characters, and then also the letters at the end.

I found many contemporary VW Passat models that seems similar to the C5 Audi A6, and so I found their transmission control modules in the same location (under the carpet in the front passenger foot-well), and so I removed and bought some of these also.

We spent maybe an hour swapping transmission control modules into our project cars, and the only ones that have so far worked as such are the ones with letter codes “BS” or “CT.”