Originally Posted by jay_bob

The EGR vacuum transducer is on the left side. The hose runs over to the manifold to operate the EGR valve. There is no direct measurement of EGR action (as in an electrical sensor on the valve), they measure the difference from the mass airflow sensor to determine if the EGR operated as commanded. If it does not see the expected change in flow then it throws the P0400.

I would check for vacuum leaks like you suspect. The climate dampers on the W210 are vacuum operated.

You can also try swapping the EGR vacuum transducer with the wastegate transducer on the other side temporarily to see if that’s your problem.