O2 sensor is a piece of cake on the 126.
First run the front end up on a set of ramps, let it cool off.
Start by pulling the passenger side carpet and false floor. At the lower left corner will be two connectors, one round the other rectangular held by a clip. Disconnect the two. Then carefully pull back on the side panel that covers the hump, the wires pass through the tranny tunnel about 6-10 inches back. be sure to push out the grommet as well.
Once that is done the work is under the car.
Remove the crossmember that crosses under the torque converter - 6 17mm bolts. Then just loosen the two front tranny pan bolts on the right side of the pan. There is a heat shield on the right side that the O2 sensor wire goes behind. Don't remove those two, but just loosen enough to remove the wire (13mm).
Now you have clear access to the O2 sensor mounted on the front edge of the cat. It takes a 22mm box wrench to remove/install. You could cut the wires to the old sensor, or feed the wires through the box wrench (what I do just in case???).
To install you HAVE to feed the wires and grommet through the box wrench. Tighten the sensor, route the wires behind the heat shield, tighten (gently) the tranny pan bolts, feed the wires through the tunnel and seat the grommet. Install the cross member and connect the lead at the lower left corner. reinstall the floor and carpet and you're done.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.