You may well have a problem with the sensor not being read properly. Your converter may be shot. Your fuel distributor may be hosed. Your ECU may be brain-dead. But more than likely, you have a fairly basic problem causing your rich mixture. In the absence of good information, I suggested some simple and inexpensive tests to try to rule out the most common problems.
But do go back and read Steve B's posts on the limitations of the O2 feedback loop before you buy a new cat.
And good luck, I'm outa here.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe