what is the remedy for a fault code 0893 (Hydraulic Fault in Tramsmission )

What is your question?

My 2007 CLK550 won't shift past the 3rd Gear. My son borrowed the car and it happened while he was driving it. There was no limp mode message or Check engine light on. Took it to a mechanic and they found the fault code 0893 Hydraulic Fault in transmission (Disengaging shift element does not disengage). They topped up the trans fluid to where it should be but it did not fix the problem. They are now saying that I need a new trans. Please help. Thanks. Raphael