Take a look at module DEKON 40.10 (Configuration of power stage diagnosis) in the ME 7.3.1H Alfa 2.0l BTS 9/633.3;16 Function- and Data Sheet. It doesn't explain it particularly lucidly, but I think 'endstufe' (i.e. output) stage is to do with a checking whether the output from a sensor (e.g. lambda sensor) is valid or sensible. Power stage diagnosis seems to be relating to the health of actuators (e.g. secondary air valve, etc.).

TTQS