Subaru Forester 1999 , 10900miles, P0420

 

Check engine light is on. Sometimes, it is off by itself, but it is on again after several times driving. the code is P0420.

 

 

I cannot find any trouble in driving. So, my friend told me maybe the rear O2 sencor can not work effectively.

 

Do your guys think he is right or have better idea? I have to pass the emission test in a month. So, thanks very much.

P0420 is catalyst efficiency below threshold. Sometimes if the rear o2 sensor is weak and is not responding as quickly as it should, it will throw this code. Have you been regular on your maintenance? Not changing plugs, oil, filters, etc.. will cause the engine not to run as efficiently, causing the catalyst to have to work harder to clean up the exhaust, which eventually causes it to not work as well.

 

What type of fuel are you running? I would suggest Chevron if you don't already run it, I've seen this code go away by switching to a good quality gas. You can also try a bottle of Techron, sometimes that helps. You won't notice any performance difference with a bad cat, unless it's plugged. I'd start with switching brands of gasoline for a few tanks and see if the code goes away. If that doesn't help, you can take it to the dealer and have them plug the laptop in and drive it and that will tell you whether it's the cat or the sensors. Be advised, catalytic converters are NOT CHEAP however if your state has smog testing, you might have to just bite the bullet and replace it.