I have a 97 e420. It is very tight to get a wrench up there, let alone have enough lever to losen the sensor. Any advice? Should I bring it to a muffler shop and have them drop the whole exhaust?

__________________
2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)

Yeah, if it's that tight you may have no choice but to drop the pipes down a bit. So, are you experiencing symptoms of poor sensor performance?
Or is it just a maintenance thing?
12 Years seems like time to redo the sensors, but in my experience they last indefinitely if the motor is in good shape and doesn't foul them. If you can access the system, and monitor the reaction time, you can pretty well diagnose them.

DG

I changed the front O2 sensor on my '95 E420 and the mechanic dropped the exhaust sydtem. He said it was the best way to get at the sensor.
Anziani

'93 300CE 170K (just bought it)
'95 E420 115K