You said: "the same fault comes back".
I assumed that to be the exact same fault code (number).
What you mean is that the symptom stays the same, you don't know if it's the same fault code each time.

Get the codes read again. If it's the same fault code (number) then have them take a look at the so-called "Freeze frame" data to determine whether the boost is too high or too low. Depending on that a trained mechanic could search for the fault.
You may want to use another garage for that task.