Max runs rich - O2 sensor?
 

My max has been running rich, barely passed emissions had high HC. I checked the computer in mode 2 to see if 02 sensor is functioning, both red and green lights went on & off together, I believe this means it is OK. When I pulled plugs all 6 indicated running rich, black with light carbon deposits no wear. Car has 116,000 miles and is 92 GXE with original cat and O2 sensor. Engine runs well other than this. Any help will be appreciated.

Never ever use octane boster as those things could easily causes detonation problems.

If you're using any feul additives, try stop using it for a while as it could possibly damage your o2. If not, then it could be possible that you're using high octane gasolines from a particular vendor that contain too much additives. Also, try not using octane rating higher than 93. Normally, the higher octane gasolines got more additives to it. Check your advanced timing setting to make sure it's not overset to 20 and above or it could be causing you to consume more gasolines than normal and also detonation problems.

I had similiar problems too after using gasoline from a local Mobil station, I don't know what the heck they putted in their gasoline but it keeps causing my gasoline light to goes on even when the tank is full. So I stopped using it and when for Sunnocco and Citco, but the light goes on and off once in a while. I hoped it didn't damaged my O2 sensor cuz that thing gave me a hell of a time trying to pray it off and it still won't move. Worst comes, I might have to lower my advanced timing down to 10degrees and go with 91octanes instead of 93.

how could octane booster cause more detonation

i always thought the high the octane fuel the less likely to dotonate thats why when you advance your timing your sopose to use higher octane fuel

That's a good question but from what I've heard, octane booster usually causes the engine to burn more aggresively than necessary and it causes problems to the spark plugs. Thus, results in detonation problems. But I could be wrong on that since that's what I was told by several experts. One thing I know for sure is Octane boosters are all worthless.

Higher octane gasolines that you get right from the gas station without using additional octane booster are all good on the engine. But the problem is that they contain a lot more fuel additives than lower octane gas. The problem could come in if you had just previously used a lot of additional fuel additive, i.e STP Silver bottle. Too much additive will cause the engine to burn rich that's all.

Thats a mith concerning the amount of additives a certain grade of gas has over another. They are virtually identical.

Bill, if your 02 sensor has more than 60K miles on it, then replace it. I hope you've already replaced the spark plugs.

As people have already stated, Replace the O2 and plugs.
also pull your injectors and have them flow checked you may have a clogged pintle that is allowing too much gas to come out.
Nissan injectors are sh!t !

O2 time
 

as they have been saying: replace the O2 and plugs if you haven't already. check indidvidual plugs for excessive burning deposits. if some plugs look fine and others are too black looking, you may have bad injector on that cylinder, ie, too rich. i had such problems a while back. because injectors are pricey, i just replaced the ones that were bad eventhough it is best to replace all at once. change O2 sensor anyway as it is overdue for you. but don't get aftermarket one from autozone as it may not even function. get one from dealer and pay extra. at least it will work.

95% of the o2 sensors for our cars are NTK in the box regardless of whose name is on the box