A couple of things to try:

- try connecting the ground wire. Maybe this sensor needs it to be connected to work

- put a meter between the black wire and ground and see what voltage the sensor is putting out with the engine fully warmed up and running. It may be that the sensor is working fine, but something is causing the mixture to be rich or lean to the point where the computer turns the light on. If the mixture is in the ballpark, you would see something like .5V from the sensor.

If the voltage is low (lean) or high (rich), you can try to adjust it with a dwell meter or a meter with a duty cycle setting. There are dozens of posts on how to do this. But unless somebody screwed with the mixture, the problem is likely to be vacuum leaks, leaking injector seals, bad injectors, crud in the fuel system, etc.

__________________
Chuck Taylor
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