1. O2 sensors, over time, get "lazy", i.e. rather than sweeping, say 5 times/second between rich & lean, they may slow down to 1 or 2 times/second.
This is called " cross-counts".
So, watching the sensor toggle rich/lean is one part, how fast this happens is another matter.
O2 sensors are considered part of a major tune-up. It is as simple to change, as changing a sparkplug on an engine.

A dripping injector would best be isolated by pulling the sparkplugs, and examaning them very carefully.
If one is noticeably different from all the others ( indicating too much fuel ), that would be the injector to suspect.

If you have a fuelpressure regulator with a vacuum line attached to it, pull it off with the engine idling.
Your fuelmixture should go extremely rich instantly, you would hear the enginespeed change quite drastically.

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2007 C 230 Sport.