I got a new scan tool for Christmas and have been checking out my o2 sensor readings for the fun of it. My readings after warm in cruise show the following: 2007 Altima 3.5

O2 sensor 1 bank 1: approx .45
O2 sensor 2 bank 1: approx .6 to .7

O2 sensor 1 bank 2: approx .8 to .9
O2 sensor 2 bank 2: approx .5 to .6

I was wondering if those numbers look about right? I wondered if the #1 bank 2 was reading a little high? No codes. Thanks.

The front (A/F) sensors on an '07 should be 1.5V types, they'll read 1.5V at stoichiometry (14.7:1 air:fuel) with lower being richer. Rich is normal if you took those readings at idle, but that's quite rich for a warm cruise. Was this on a really cold day with very low intake temps?

Nominal is around 0.7V on the rear O2's with the cats lit, so those readings are pretty healthy considering the engine is running rich on both banks.

It was about 43 degrees Fahrenheit (6 deg C). Intake temps were about 55. Not sure if that's considered cold. What should the numbers be on the fronts approx? Can you think of any reasons they would be running that way. Thanks for your input!

That's pretty cold by engine standards. Keep in mind that injected engines don't have chokes like an old carb engine, so they can't enrich arbitrarily just when it's cold. The ECM reads the temperature and enriches the mixture as necessary. That's why my wife's 2.5 Altie loses about 2~3 highway mpg in the winter. Everybody's does. If you check again on a summer day you'll see higher voltages on the A/F's because the toasty air lets the ECM lean the mixture. I actually put a thermostatic stove from an old Chevy on the intake of my departed '04 Hyundai to suck air off the cat in cold weather, and that engine got about the same mileage year round. I always kind of wondered why Nissan and other OE's don't do something similar. I guess it doesn't matter to them because EPA doesn't test in winter temps, but it would save customers winter $$ and would be a cheap addition in the kind of gazillion-lots they buy parts.

Okay that makes sense. It's interesting to mess around with the scanner, but I can see how it could lead me down a rabbit hole! Are the rear o2's the same type as the front? As in the 1.5v?

No, the fronts are wideband and the voltage is a direct correlation with the amount of O2 in the exhaust stream. The ECM uses feedback from them to evaluate combustion efficiency. The rears are traditional HO2S's and the ECM has to evaluate both voltage and response time in order to tell what's going on. The reason they "swing" once the cats are lit is because the ECM deliberately creates cycles of slightly-rich and then slightly-lean to see how fast the sensor follows. Slow response plus low voltage indicates a bad cat. They're strictly there for the government and not the engine, their sole purpose in life is to tell you if your cats are healthy.

Great info, thanks!

I've noticed the b1s1 has a lower voltage than b2s1. I noticed the same thing on my 06 Pathfinder with vq40. Do you know why b2s1 has a higher voltage reading?

I've never fully explored that. I used to think it might be that Bank1 ran warmer on the transverse VQ's because of the difference in airflow, but then I noticed most of the axial VQ's seem to do it as well. So the answer is, I've noticed it but I have no idea why.

Not to hijack this thread but how do I rule out the exhaust cat over 02 sensors? I literally just got a p0420 code on a trip uptown. I loaned the car to my nephew for around a month and swear it came back with an exhaust leak. Of course I was driving my new one and thought maybe I’d gotten spoiled.

Normally P0420 by itself means a dead or dying cat, but the one thing that can fool the ECM about it is an exhaust leak behind the front cat or anyplace near the downstream O2. That will suck fresh O2 into the exhaust stream when the engine Miller-cycles, making it appear to the ECM that the cat has lost efficiency. So your suspicion about a small leak may well be correct.

I’ll get it in to my exhaust guy this week. I reset the code and it lasted for about 2 weeks. The weather got chilly again and it kicked back on. My son was driving it while I was on a business trip for 2 weeks. It looks like it’s used a few ounces of oil all the sudden so I hope the cat didn’t crumble and dust the engine.