Subaru Repair: Check Engine Light - Nissan Altima, 2002 nissan altima, nissan altima

Question
I have a 2002 Nissan Altima in good shape and has about 123K miles on it. I'm a driver (smile). My check engine light keeps coming on and the code is 0420 Catalyst Inefficiency below threshold. I changed the top front o2 sensor because my mechanic said that was the problem. Then my car was running pretty good. After about a month the light came back on with the same code. Before changing all of my o2 sensor, I wanted to find out could it be the catalyst converter that's going bad. Please let me know the next step to take before I go throwing out all kinds of money.

Answer
Sounds like that is what the code says. That code simply means the front and rear 02 sensors are reading about the same.  The rear sensor should read much lower than the front sensor, that shows that the catalytic converter is cleaning the exhaust like it should.  I would have the cat replaced. You may want to take it to a muffler shop and see if they can put you a universal converter on.  If they can I would really save you some big money!  Hope you get it fixed!