Yes, access is under the car along the exhaust pipes. Looks like your car has 4 different o2 sensors, 2 ahead of catalytic converter on left and right and 2 after the converter left and right. You need an OBD reader to tell which one may be out. Find the part with the online parts seller FastLane. I would suggest going with the more expensive version that has the correct length wire versus the universal sensor where you need to splice the new sensor to the old plug. IMO the splice mechanism is not well designed and was not easy to do. If you have an ATF leak in the vicinity of this splice, the ATF will eventually worm its way into the connection and the splice will fail. YMMV.

glenmore
1991 300CE
2000 C280