Hello all...

Just returned from a MB dealer diagnostic with the following error results:

1. Hi voltage for rear O2 sensor

Code PO138- Circuit High Voltage and Switchover valve controls
Recieved "Mixture too rich" fault.

2. Upshift delay- My car shifts aggressively in 1st and 2nd gear, moreso
when cold. Dealer indicated that is may be Throttle actuator, whatever the hell that is....

I was informed that the O2 sensor needs replacing...Please educate me on where the O2 sensor is on a 1994 C220 and if this is a difficult process to replace. Thanks in advance.