Howdy,
New to the forum but have been doing my own stuff on cars for years. I the case of my 12 C300 4matic I bought it CPO under a 8-80 in 12 and had issues for sure but all were addressed at the dealer level. Fast forward to the 96K mark, out of warranty and realistically probably worth about 10K the last place I want to end up...voluntarily...is the dealer. Love Merc support , this is purely economics.
So had a few issues. In short order I had a limp mode event that pointed to the output speed sensor on the dreaded conductor plate (0721 trans output speed not plausible) I could reset it and on occasion it would be good for 30 odd miles and then finally come back so I tore the valve body and conductor plate out and sent it to a facility to have all three sensors changed WITHOUT viginizing (clearing the software). The other issue was a wheel speed sensor that was lighting up and shutting off the ABS/ESC etc. Nothing pointed directly too it and I needed to roll around with this a bit before recognizing the issue in live data (no rpm registering when moving)

To my observation: . Post trans reassembly I was pretty confident the software was gone. I sent out a mayday on the 204 sedan forum and was searching for a indie to code it after a tow to the shop. I was waiting for some calls back and started playing with my code reader, going through the various menus when I recognized a trend. In EACH instance the main trouble code set off additional codes in different sub systems. In the instance of the conductor plate/Trans stuff the reason I wasn't seeing the software was that when the trans coded it tripped other codes in the engine menu (suspect the relationship with the ECU and the TCM here) and the ABS/EBD menu (limp mode, 2nd gear and reverse and ABS/EBC disabled). So clearing the tranny code, and seeing the software required me to recognize that and clear all the other codes (on the icarsoft MB2 that is going through the different menus for each system) and OPEN the CAN communication lines back up between the TCM/ECU/ABS/ESP systems. Once I cleared my ENGINE (ECU) and ABS/ESC codes from their respective menus everything appeared and cleared.
The same story with the wheel sensor. Primary "C" codes in ESP/ABS menu but then D409 wheel speed implausible (no wheel designated) in the "ENGINE " menu of all places and then a 1425 code..CAN BUSS related under the "TRANS" menu. ALL OF THIS for the wheel speed sensor! For sure the only vehicle I have seen that casts such a wide and protective net when a fault occurs.
So you have I'd a issue, put in a new part and cleared the likely code but aren't seeing it solve the issue. BROADEN YOUR SEARCH ZONE and assume that a code, particularly if its associated with brakes/engine/drivetrain has ALMOST CERTAINLY reached up to the Engine/ECM at the minimum and caused a fault there as well. So before you think you were wrong and start throwing money around dig a bit deeper.
Good luck. Matt