This is the oxygen sensor. It is easy to test with a high impedence volt meter, such as a DVM. You can Google for "+test +oxygen +sensor" and get good discussions. But briefly, check the voltage at the O2 connectors. In the older cars, like by '89 760, the one before the catalyst will vary from about .2 to .8 volts DC if it is good. The car must be running and the exhaust hot.
Ask the mechanic to test it, not just do "see if this fixes it?"

I am not familiar with this engine but the theory should be the same.