About three months ago my car was throwing a p0420 from the front left "bank" and I had bad gas mileage on my car. In late December I replaced the the front left o2 sensor and my gas mileage was normal/good again. A few weeks ago the check engine light came back on and it's giving the same code again and I'm pretty sure it's getting less gas mileage again. However there were a few days (+ or -) where the light turned off and my mileage was normal, but then it came back on.


I need help diagnosing the problem. I'm hoping it's just a different o2 sensor and not anything more expensive.

The guy at advanced that read the code printed it out for me and it says "Definition: Catalyst efficiency low-bank 1

I'm across the country and I have a long drive back so I need to get my gas milage back to normal but I have very little money to spend. I need the cheapest/most reliable "fix".

I've been told that when it throws the p0420 the car goes into a sort of "safe mode" and runs rich, which would be the cause of my lower gas milage. Is that true? If it it wouldn't it be fairly safe to assume that since the front o2 sensor was just replaced recently that it's likely the rear sensor now?

 

So what happened with your code? I have the p0420 code and have turned it off with my scan tool. And it came back on. It sounds like I have and exhaust leak but can't find where. The cat was replaced I think at most a year Ago. Well it just turned off by itself today!! And wow she runs great. I bought a pry bar to pry on the exhaust bc you can only hear the leak when the car drives. So I'm just wondering what happened?