2000 Toyota Corolla VE Trouble Code help???

Hi all

My dad’s car is throwing a check engine light code. I got it checked out at a parts store. Code is PO171 which according to Hayes Manual is “System Too Lean (Bank One)”

My choices are:
1) One or more injectors are clogged.
2) Oxygen sensor is bad
3) Any others you can think of?

Now, there are two Oxygen sensor’s on the car. One is before cat and the other is after the cat. According to Hayes manual, after the cat Oxygen sensor doesn’t throw the code. So I am thinking it’s the Pre Cat Oxygen Sensor which is also known as Main Oxygen Sensor.

Here are my questions:

1) I would like to take out my oxygen sensor from underneath the car (It is located on the exhaust piping right under the exhaust manifold I believe). I would like to test it after I take it out of course? Anyone got info on how to take it out and test it? Is it as simple as disconnecting the battery, going under the car and disconnecting the harness to the sensor and unplugging the sensor out?
2) If this doesn’t help, I will go ahead and get a fuel flush done on the car as final resort and change the fuel filter as well.

Anyone has any suggestions or have a FSM (Factory Service Manual) that they can lend or fax me the pages on how to do this would be very much appreciated.

Thanks
RINKU