it is a 97 s-10 - v6 - where is the oxygen sensor in bank 2? thanks

sencer 2 is behind the cat, sencer one is closer to the engin

the one i should replace is behind the cat??

Bank 2 is the passenger side. What is the full name of the sensor? If it's phrased correctly it should be Bank 2 Sensor 1 which would be the sensor on the passenger side manifold.

its a code which is P0175- System too rich Bank 2 - i have done research and decided i would start with the oxygen sensor first and then go from there

The PCM set that code based on the information that it gathered from the sensor on the passenger side manifold. It's uncommon for a bad sensor to cause that code, but it is a possibility. Normally cleaning the MAF will clear it up.

P0175 OBD Trouble Code

Technical Description

System Too Rich (Bank 2)
What does that mean?

Basically this means that an oxygen sensor in bank 2detected a rich condition (too little oxygen in the exhaust). On V6/V8/V10 engines, Bank 2 is the side of the engine that doesn't have cylinder #1.
Note: This DTC is very similar to P0172, and in fact your vehicle may show both codes at the same time.
Symptoms

You will more than likely not notice any drivability problems, although there may be symptoms such as a misfire.
Causes

A code P0175 may mean that one or more of the following has happened:
The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters (K&N, etc.) can cause the MAF to become dirty if the filter is over-oiled
There could be a vacuum leak.
There could be a fuel pressure or delivery problemPossible Solutions

Possible solutions include:
Inspect all vacuum and PCV hoses, replace if necessary
Cleanthe MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
Inspect fuel lines for cracks, leaks, or pinches
Check the fuel pressure at the fuel rail
Check the fuel injectors, they may be dirty. Use fuel injector cleaner or get them professionally cleaned/replaced.
Check for an exhaust leak before the first oxygen sensor (this is unlikely to cause the problem, but it is possible)