Need advise on code 420. A few months ago my CEL came on and I checked the code to be 420, "Catalytic system efficiency below threshold". So I'm guessing it was a bad O2 sensor. I was going to order a new O2 sensor today but on Friday I noticed the CEL light is no longer on? The only difference I can think of is I just filled up my tank with 76 gas and normally I use Exxon or Shell. Could it of been the gas I was using? The 4Runner has 162K miles on it and it runs great. No issues even with the CEL on and gas mileage never changed. Should I even bother to change the O2 sensor? I know its about time I should, especially with 162k miles on it. How long do O2 sensors last anyway?