2000 Nissan Maxima 02 sensor location

Can't tell you exactly because you didn't provide enough information, but I can give you some general guidelines. There is no 3.1L for your car but there is a 3.0L. You could have California or Federal emissions system. You didn't say if it's an "upstream" or "downstream" sensor or why you think it needs to be replaced.

Upstream sensors are usually screwed right into the exhaust manifold, but they could be found in the exhaust pipe before the catalytic converter. You have two. Some suppliers call them the "left" and "right", and some call them the "front" and "rear".

There's also oxygen sensors after the catalytic converters. Those are the downstream sensors. They'll be screwed into the pipes and there can be only one of those where the two pipes join together, but your car uses two, so you have four oxygen sensors all together.

You didn't say why you want to replace a sensor, so be aware they don't fail real often. When there's a diagnostic fault code referencing one of the sensors, you have to rule out a lot of other stuff before condemning the sensor itself. Most failures are caused by something else, usually too much raw fuel going into the exhaust system, and the heat takes out the sensors. Those possible causes have to be checked first.

My service engine is staying on! And I checked, and possible it is the trouble? Also, replaced the gas cap!

Also, thanks for your reply!

First you gotta have the diagnostic fault code read, but you have to be aware they never say to replace parts or that one is bad. They only indicate the circuit or system that needs further diagnosis. When a part like a sensor is referenced in a code, it is only the cause of that code about half of the time. You also have to check for wiring problems.