kaine : Hi. I must preface my response by acknowledging from the outset that I'm NOT an expert on your Audi A4 (which was built on a Group B platform).
However, I fool my self into the illusion that I do know a bit about Coding on VAG vehicles and I do have a working knowledge of the mystic arts of hexadecimal (look it up) mathematics.
So, it is with these caveats that I reply - and because I happen to like the embedded humor in the way that you write stuff!
OK - lets look the process that you have described for this tweak (and my apology, but I must launch into techo-babble from this point)- the value of your existing long-code string =
0205115. If you look
HERE, you will find an explanation of the purpose of each of the digits in your long-code string.
On the Equipment 1 section (0xxxx?x) - which refers to the second last digit (as per your post), the software switch descriptions are:
+1 = Standard/Basic Value - Add to the Sum of Options below
+1 = Audi Music Interface (AMI/PR-UF7) installed
+2 = Satellite Radio (SIRIUS/XM) installed
+4 = Analog Input (AUX/PR-UF0/UF1/UF2) installed
Conclusion (thus far) = yes, adding 4 to the second last digit is indeed correct!! What the change does in reality is to switch the hex37 module by disabling AMI setting and enabling AUX.
So, everything looks OK up to this point in your tweak instructions - BUT - the odd thing about your long-code string is the first Byte (i.e. Byte0 - which refers to the first two digits). On your car, Byte0=02.
On the Model section (0?xxxxx), the meaning of the numbers are:
1 = Lamborghini
2 = Audi A3 (8P)
3 = Audi A4 (8E)
4 = Audi A4 Cabriolet (8H)
5 = Audi A6 (4B)
6 = Audi TT (8J)
7 = Audi R8 (42)
You say that your car is an Audi A4, but according to Byte0 value, your hex37 module believes that your car is an early model Audi A3 (8P)!!
The module's belief that the car is an A3 is further confirmed by the value of Byte1 (digits 3 & 4) on your long code string which is 05. Again based on the table in my link:
0xx?xxx: Sound Characteristic
0 = Sound Characteristic linear
1 = Audi A4 (8E)
2 = Audi A6 (4B)
3 = Audi A4 Cabriolet (8H)
4 = Audi TT (8J)
5 = Audi A3 (8P)
9 = Sound System
"5" means Audi A3 ( not Audi A4 - which is "1")
Very strange - was the hex37 module in this car from the original factory build, or was it retrofitted (maybe from an Audi A3 donor car)?
Now, again I stress that I'm not an A4 expert. I'm simply making and observation, rather than providing advice.
Of course, you can throw more money at this problem by buying an OBD11 device- but if you proceed with this option, you will need to buy a PRO license to change long-code values!
However, before you reach for your credit-card, I suggest maybe that you investigate the anomalies that I identify above. Perhaps, the failure of your current diagnostic device has its genesis in a problem with the underlying long-code string value for the module - maybe?
Don