I threw a code the other day that means 1 of 3 things...
1) bad front O2
2) bad rear O2
3) bad cat
Guessing it's not option 3 because I only have 50k miles on the car, and prolly not option 1 because gas mileage hasn't suffered. What is the procedure for testing if an O2 sensor is bad? Have volt meter and ready to check it out, but...car running or just the key on? Where do I put my leads? Any help on this would be appreciated.

 

I think it will be easier with a scanner! I don't know what are the correct voltage readings you have 4 wires to check but i will compare it with a good 02 sensor