2003 Honda Accord LX 2.4 liter. I had an O2 sensor heater circuit engine ligjt for quite some time. I finally replaced the O2 sensor, and now I’m getting a P0420 engine light code (low catalytic efficiency) after a drive cycle. Is there a chance that my new Walker brand, O’Reilly auto parts O2 sensor is bad, or is it more likely my catalytic converter has failed?
Dollars to donuts it’s your catalytic converter—the car is the right age for the cat to be giving up the ghost. You should replace it with a new cat, though with a car this old you could probably go with a less-expensive aftermarket one, since the car’s probably not going to be around for the 20 years a $700-1200 OEM cat would last.
That said, the REALLY cheap ones—like >$100 on Amazon—literally only last a couple of months. (Don’t ask me how I know.)
Driving it like it is now won’t hurt anything (except the environment); it just means you won’t pass a smog inspection if your jurisdiction requires them.
If you’re reasonably clever with basic electronic stuff (and also a slightly ethically dubious person), you can solve this problem permanently for a few bucks by hacking the downstream O2 sensor circuit. This is illegal and probably morally wrong because it just hides the fact that your car is out of compliance with the smog laws, but if you Google “P0420 O2 sensor hack” someone will tell you how to do it.
I do have to pass emmisions unfortunately, and I’m about 2 months overdue. The car has almost 300k miles, so I’d be happy to just get one that gets me another 2 years of registration. Might go a little better than an El Cheapo Amazon cat though.