My car has over 200k on it and as far as I can recall it's running the original O2 sensors. I consistently get 23mpg/tank with basically all short trips (20min or less) at 70-80mph.

I have an OBD sensor reader (ultragauge) and when I monitored my o2 sensor voltage, all 3 fluctuate between 0.001v and 0.8 or so, but I've read that it is supposed to be between 0.2v and 0.8v but I'm not sure if that's under load or not. When I take my foot off the gas at high speed they all get down around 0.01v but I'm not sure if that's normal or not.

The car has had mostly highway miles and nothing but 93 octane its whole life and I haven't replaced any other sensors so I wasn't sure if a lazy/old O2 would show a very obvious mileage decrease or not.

 

Obvious question of the day...
****** (drumroll please) *******

Any codes??

 

I feel the quote in my sig has significance here... (pun intended)

 

That quote is great! See my sig, it applies as well.
I recall a saying, it goes something like this..

IF IT AIN'T BROKE LEAVE IT ALONE.

 

I like your quote, I am 27, not super young, but no 40 either.

 

lol - no codes - or sexual arousal - rockauto had some good prices on O2's so it made me think about it...

 

That is normal. And the voltage drops to 0 when youre coasting because the ECU cuts fuel. Less than .45V means leaner than 14.7 afr, higher than .45V means richer than 14.7

Your sensors are good.

As sensors get old, they start to respond slower. the ECU will tell you when theyre too slow

 

Thanks - that's good info - I thought they'd only throw a code if dead (no voltage).

Pretty impressive since average lifetime is supposed to be 100,000 miles (for a heated sensor).

 

NOPE. Tired O2s don't throw codes.
If you have old O2s, replace 'em. And you only need to replace the primary one.

On the 99 MAxima its the one right behind the radiator in the exhaust manifold. In cars like my 98 and before, the primary is further down that exhaust section, near the bottom of the Y, so to speak.
The 4th gen ECU only uses the primary to adjust air/fuel mixture, and a tired O2 gives bad gas mileage.

Ideally, you should change the O2 after you've done ALL other regualr maintenance, like plugs, air filter, fuel filter etc. If, after all that your car still gets bad gas mileage, then it's the tired O2.

 

Aren't there 2 primaries for a 4th gen (aka the upstream sensors)?

 

Yes, 2 primaries and one secondary in my 96 Fed.

 

There are 2 primaries but the ECU only uses the front one for air/fuel. It's in the FSM.

 

The FSM actually says both left and right bank o2's control injector pulse. The only one that doesnt is the rear downstream sensor. Replacing only 1 upstream is only completing half the job