O2 sensor basics and code p2195

[sspanzer]

Car:
06 Solara v6 sports SE
With code air intake mod. I didn't install the intake. The previous owner did.
My gas mileage is not great ever since I bought it. Even on high way 15 gallon is about to run only 330 miles.

My engine check has been on for several months now.
It reads p2195, which suggests it might be the O2 sensor problem, or some leaking in the intake, or even the intake sensor problem.

My questions:
- Is it likely a O2 sensor problem in general? How many O2 sensors are there for my car? I can only spot 2 of them.
- Has anyone done replacing the O2 sensor themselves? It doesn't look like a hard job, and I found some youtube tutorial. Anything to keep in mind?
- I saw different types of sensors sold from autozone, some are for "before catalytic converter" and some are for "after catalytic converter". How do I know which one is for which one, before or after CAT? Are they both bank1?

Thanks so much in advance.

[1gSE]

Car:
.
My questions:
- Is it likely a O2 sensor problem in general? How many O2 sensors are there for my car? I can only spot 2 of them.
- Has anyone done replacing the O2 sensor themselves? It doesn't look like a hard job, and I found some youtube tutorial. Anything to keep in mind?
- I saw different types of sensors sold from autozone, some are for "before catalytic converter" and some are for "after catalytic converter". How do I know which one is for which one, before or after CAT? Are they both bank1?

Thanks so much in advance.

Its an o2 sensor code, nothing on the intake side. You have 3. 1 in each maifold and one behind the cat. The hardest part about replacing one is getting to it and breaking it loose. Its pretty much plug-n-play...and some screwing. The one you need to replace is in one of the manifolds, I'm gonna say the rear. I'm not exactly sure which yours uses, there's oxygen sensor and air/fuel ratio sensor. I don't know if there's fed or cali spec for gen 2's so I won't tell you which one to buy