Check engine light ON

I have a Mercedes Benz, model C 280, year 1994
The engine started to shake and the check engine light came
on. I took it to the mechanic for a check up, and the mechanic
toll me he need to change the engine control unit, so he did
according to him. He called me the car was ready and when
I drove the car out of the shop, the check engine light was on
I returned the car to the shop, and this is the 3th times it
happened, Now I think he does not know what to do any more
Have you hear something like this before?
By the way the car is running well, but the check engine soon
is on.