A few months ago my engine light came on.. I went to Auto Zone and used the ECU reader.. it said PASS -- no issues and I ran it a few times -- then the engine light turned off.. so I figured nothing was wrong..

well yesterday the light came back on..

I went to Auto Zone and got the ECU reader again..

The first time I ran it the error code said

P0420 Catalyst Threshold Below Efficency (Bank 1)

I ran it again -- same issue

I went inside and the guy said it was the O2 sensor..

I went back out and ran it again..

This time it said PASS no issues -- twice??

So do I have a problem or not??

I have a 2000 max with 28k miles on it.. and I use premium gas only..

 

you and i are in the exact same boat. we even have the same cars. the only difference is, is mine is a gxe.
anyway, this code is very common amongst the 2000's. apparently its an error in the ECU. it most likley is not the O2 sensor, or so ive been told. if you take it to an honest nissan dealer (if they actually exist) they'll diagnose your car for $85 and then waive the fee, IF the problem lies within the ECU, since this is covered under the 8/80,000 warranty. i say, dont worry about it. unplug your battery, reset the ecu yourself and continue driving.

 

Thanks..

I wasn't sure if I should replace it just in case???

My 97' max had the same issue.. so it makes me a little leary when my new car has the same issue ya know???

My 97' actually did have a bad rear 02 sensor -- because when we took it off it was in horrible shape.. all worn out.. but the car only had like 70k miles on it.. and it really shouldn't have even been bad at that time..

I dunno.. I guess I will wait and see what happens..

 

http://maxima.theowensfamily.com/tsb/NTB00-070a.pdf

that's the TSB. I actually have a similar thing with the light AND the code going away and not coming back for weeks. Anyway, once it the SES light comes on again, go read it at autozone to make sure it's 0420 and then go straight to the dealer with a copy of the TSB (link above). They should take care of it under the warranty. If you're out, ask how much they charge.