I got this code P0420 at AutoZone, Said catalyst system efficiency below threshold. They told me it's probably the O2 sensor going bad. Is there anything else that this code could mean?

 

Usually it means that the HO2S1 and HO2S2 are giving the same or similar readings. This means that the catalytic converter is not working. It seems to me that whenever a Check Engine light comes on, the O2 sensor always gets replaced. I would test the O2 sensor, but you need a more advanced tool than a simple code reader. If you have a scan tool that can read live data then you can do this test. Pretty much take out a vacuum line from the engine and look at the O2 sensor, if both sensor 1 and sensor 2 go towards 0v (lean mixture) then the O2 sensor is working fine. You can also spray some carb cleaner through the throttle body, this should cause the O2 sensor to go to 1v. If both O2 sensors are similar in readings (especially when you cause a change), the cat is bad, not the O2 sensor. It could also be that your O2 sensor is bad, but my opinion is that it will be the cat.

 

99.9% bad cat

 

How long is the cat. converter supposed to last? I got 85800 on this car. Seems a bit soon.

 

call Honda dealer and ask them if it is under warranty,,, I know that honda extended the warranty on cats but i think it might have been only up to 80,000 miles..

My advice.... call Honda and see if they can help you out ,,

 

It's 8 years or 80,000 miles. He's just a little over, may be able to get a goodwill replacement. (or at least have them pay for parts)

 

Does that mean that until I get this taken care of the fuel mileage for this car is gonna be off?

 

Nope, the cat doesn't have anything to do with the fuel economy. You won't pass emissions but i don't think that your fuel economy should be effected.

 

This code doesn't come back right away, after it was reset by a scanner, it came back after about a week of driving. Does that give any indication on what the problem is, the O2 sensors or the cat. converter?

 

There is a certain criteria that the car has to meet for a code to be set. Like the fault has to occur 3 times in a row when the car is running for 45min bla bla bla. Now if your car hasn't met the criteria in 2 weeks, the code won't set even though the problem is there. I think i already posted how to tell if the cat is bad or if the sensor is bad.