Nissan Repair: 2001 Nissan Pathfinder failed emission, competent mechanic, nissan pathfinder

Question
Hi,
My 2001 pathfinder just failed the emissions test. the codes were P0420 and P0430 Below threshold band 1 and 2 respectively.

I have an appointment with a mechanic but would like to know what to expect.....or at least some idea.  
I had the MAF sensor replaced recently ... but the SES light would not go off. The dealer, where I had it repaired, didn't have an answer for me. He said it was a common problem with this year of nissans.

What are your thoughts?

Answer
Hi Jill -

There are many reasons why vehicles can fail emissions tests. The most common cause is a faulty Oxygen sensor (OS). This sensor monitors emission levels in the exhaust manifold and reports back to the car computer. Although the emissions may be normal, the OS could be faulty, and thus sending the wrong input to the computer, lighting up your dashboard like a Vegas evening. Fortunately the OS is not too painful to replace.

It appears that one of the many sensors that report to the computer is causing it to change the emission settings. Taking it to a competent mechanic and having them put it on the diagnostic tester will definitely find the problem, but could be costly.

Ensure that what sensor they replace is exactly what is wrong. Some mechanics "guess" and before you know it, you are paying for things that did not need replacement in the first place.

Hope this helps -
calvin