I got the dreaded VSC Check Engine and Trac Off lights on my 2005 xB the other day. I ran the car over to Autozone and the Code P0420 (Catalyst system efficiency below threshold - Bank 1) came up. Plus I notice my gas mileage has dropped SIGNFICIANTLY.

My question is how do I know whether is a bad cat converter that needs to be replaced or just an O2 sensor? I know I can do the O2 sensor myself so I am hoping it's that but don't want to replace a part that doesn't need replacing if it's the CC.

Any help anyone can provide would be great.

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Wouldn't the bad front O2 sensor set its own fault code?

 

You can take it to a muffler shop and have them check the back pressure (takes them about 5 minutes). Around here (central Oregon), they will check it for free with the understanding that you will have them install the cat. So pin down the price of the cat up front and then make sure you have some assurance that your mpg will greatly improve once the work is done. I can't remember why, but I believe the O2 sensor often needs to be replaced with the cat, but not always. Not sure how you would know if they are being honest on that one. Someone here probably knows.

I know many have paid more, but you should be able to get a new cat installed for under $400.

 

A bad rear O2 sensor would set that code, as would a failing cat. A bad rear O2 sensor wouldn't affect performance or fuel economy however, whereas a failing cat would. The rear O2 sensor doesn't affect the fuel trims, only monitors the efficiency of the cat for emissions compliance purposes.