What do you think of these o2 sensors? I cleaned them and reinstalled them and the cel came back on.

 

Whenever there is any doubt about O2 sensors, just replace them. You will get better power and better gas mileage .

 

there are different ways an O2 sensor can fail. Each sensor has a heating element and a sensing element. My bet is the fact that you found a carbon cover on the sensor is that the heater is not able to burn off the carbon and thus that's setting the CEL. Time for a new sensor.

 

The 02 sensor is inside that protective cover. You can clean it till the cows come home and it will make no difference. If their bad, their bad.