Bosch rates their O2 . . .
sensor for 60K miles, so it may be its time! They only cost about $85 at discount. As Manny says, it should be considered during a major tuneup. Even if it's not bad (open/short or heater blown), it deteriotes with time and rich/lean response becomes slow (time wise).
Also, DTC 13 says ". . . rich or lean limits" so you need to measure Lamba with an AST Retreiver via its "live data" feature. Maybe you can 'borrow' one??
Regards . . . . JimF
'94 S500 Cpe
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