$170 for the O2-sensor of a 190E is outrageous.
Both Caliber Motors and Cutter Motors (Mercedes Dealers) charge about $130 for the 3-wire O2-sensor.
Even the 4-wire type for a 1995 C36 cost me only $140.
If you really want to save $$$, get a new 3-wire O2-sensor ($50)for a (1985-1990?) Ford Mustang 5.0, splice the connector off your old O2-sensor, and you've saved yourself almost $100.
I did this with our 1988 300TE and it worked fine.
Labor-time to replace is out of line. Find a reputable independent shop ASAP.
Check connections of the O2 sensor first.
Best of luck,