destroyer

Hi guys, hoping someone can help here, my wife's year 2000 A170CDI won't rev past 3K once it gets warm. fault codes for disconnected battery and turbo (under or over) boost have been wiped from the EMU. The garage's book said the it was a wastegate sensor or a hole in a pipe. Any clues on how to find and test the sensor and which pipes to look for? I am surprised that no engine management lights have come on.
Any clues will be greatly appreciated. Steve.