Transmission code P1618 2000 C70 T5 Coupe

[dandanboi25]

Hi,
I have the code p1618 on my 2000 C70 with 270k km, apparently this is the code the ecu throws when there is an error in the TCU, transmission shifts fine but if i try limit it to 1st i get a flashing up arrow and the car seems to get stuck in a higher gear (no acceleration but drives fine at higher speeds) and does not return to normal until I reset the computer, however it shifts fine when i limit it to 1st and 2nd, I also have code P0030 however this is probably unrelated, is there an easy way I can access the TCU? any help appreciated.
Many thanks,
Dan