A couple weeks ago, I replaced the front O2 sensor because I was getting a Check Engine/ 103 code (G3/4 O2-CAT front right) on my 97 E320. After replacement, it ran great for a few days, and now running bad and getting the Check Engine light again.
Getting these codes:

300 Misfiring damages TWC
305 Misfiring damages TWC Cyl 5
306 Misfiring damages TWC Cyl 6
304 Misfiring damages TWC Cyl 4
133 G3/4 (O2-CAT front right) aging
Repeats sequence from 300

Anyone know the cause, or is there a way to test the new sensor to determine if it's working correctly?

 

I always start by resetting all the engine adaptations, and 90% of the time this fixes this fault. However, I am using the genuine Mercedes-Benz Diagnostic system, not sure how you can do this without it.