There is a reset button, located under the front passenger side carpeting. Push the passenger seat all the way back, undo the carpet snaps and you should see a box with the reset button.

BTW, did you use the Mercedes Bosch O2 Sensor, or the Mustang 5.0 Bosch O2 Sensor? I have heard from several members that the Mustang part is the same for much less $, but you have to splice the connector wires.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible