Personally have issues with people who buy special decade-old cars and can't spend less than $30 on Amazon & play store for Torque Pro and a OBD-II mini dongle. (if you have an Android phone or tab).


This lets you look at all four O2 sensors to see what is going on (rears should not move off midpoint when lit up).


BTW I buy only Bosch sensors for Mercs. Only difference I could find between front and rear is the length of the cable. Bosch 16167 Oxygen Sensor, OE Type Fitment by Bosch (upstream) Crossfire Bosch 16276 (downstream).


Now if a wiring whiz you can save a few simoleans with a Bosch 15733 Oxygen Sensor, Universal Fitment