Alright, automotive board. Have a 2010 Nissan Rogue with 145K miles on it. Have done minimal maintenance on it over the course of its life, but recently had a coil go bad and the engine was misfiring. Replaced all 4 coils, and within a week, started showing the P0420 code. Had almost the exact same thing on a 2007 Nissan Altima after I replaced the coils as well.

Is this a bad O2 sensor, spark plugs, or the cat? I was reading a previous thread on the same code for an Infiniti, so it must be something with Nissan engines. Somebody had mentioned an O2 spacer in that thread, would that help me here?

Not looking to dump a ton of money into a 12 year old car that I'll likely get rid of at some point relatively soon, but my main concern would be getting it to pass inspection.

catalyst system efficiency code.

For reference

P0420 OBD-II Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1) (yourmechanic.com)

Check upstream and downstream O2 sensors.

If downstream O2 is reading like up stream, then the catalytic converter is dying.

Park on side of road with it on jacks.

Wait for someone to steal cat converter.

File insurance claim.

Get cat replaced for free.

And ruin a thief's day.

There was a similar thread recently. I think it was an infinity. Find that thread and read my response.

Upstream O2 sensor is reading around 2.2 or so, downstream is cycling between 0.1 and 0.9.

MBL, I think you mentioned an O2 sensor spacer. I'll go back to the other thread and read that.

It might have been my Equinox that was the prior thread. eBay O2 spacer for the downstream sensor and I'm back in business, passed inspection a couple of weeks ago.

Yup. That was me. Get ready to drop some coin on new cats.

Serious question: are universal cats not viable anymore? Or just not available? Years ago I had an honest tech tell me it would be $2200 if he replaced one on my 05 Expy with oem, but he sent me to a muffler shop that welded in a universal for just over $500.

There are aftermarket universal cats for some vehicles. For the QX80, the front cats are integrated into the exhaust manifolds and tucked in there pretty tight. Thus, to replace, the whole damn thing has to come out which is why it costs so much.

The old joke about Infiniti is that it's a description of how long you will be paying for that vehicle in some form or fashion