Alright, sorry in advance for the novel here but I wanted to be thorough up front to answer as many questions to get to the heart of the issue here.

1st off, its a 2003 c240 base sedan, 187k miles. Not sure that the trans fluid ever was changed as it was pretty dirty looking but not burnt smelling or anything. I have done quite a bit of searching and havent seen anything quite like mine is doing. My understanding is these transmissions are pretty solid as long as they arent totally neglected to the point of failure or have the valeo radiator failure (doubtfully ever the case here or long resolved given mileage and age now).

Anyway, this car was picked up recently in trade and had a trans issue where it would drive and move normal until warmed up some then drop to like a neutral even though in drive or reverse and just rev...like slipping trans. I did the conductor plate, 13 pin connector, filter and 4 qts new fluid and it drove fine for almost 30 miles before doing it again now.

My question here is how sensitive are the trans electronics to dirty/particles in fluid? Can a solenoid or something on the board be not allowing pressure somewhere making it seem like neutral? Do any of the solenoids typically fail causeing a neutral like issue and not the typical limp home mode? The orings on the solenoids were good when I did the conductor plate swap. Am I going to need the mercedes specific code reader to get TCM codes or are there typically 1 or 2 places that cause this? Or is there an off chance I got a bad conductor plate or it didnt like not haiving all new fluid replacement when I replaced it? I find it hard to believe its in the transmission and not electronics due to how well it drives and shifts cold, and even warmed up to operating temp for the 30 miles before it started acting up again. Just want a direction to go without having to just shotgun parts but if I have to get a mercedes shop to read out the TCM to really get this resolved, then I guess I have to go that route. I just dont want to spend $150+ on a scanner Ill use a couple times and likely never need again. Especially working in a shop with specialty tools right there that cant read the car I have.

Thanks all and I do appologize if this is covered somewhere but I have google searched and looked through this forum and all the trans related issues are the typical limp home and shifter stuck typical ones and mine is just a bit different. Just dont want to condemn the transmission assembly if its not the real issue and it just gut feeling seems like its electronic.