Damn o2 sensor

[PRIMAJIN]

i drive an 04 sti, the code read wen the CEL came on was
P0171 system too lean bank 1. Out of the two o2 sensors on the car, how do i find out witch one needs replacing? i need help with this one, thanx alot fellas

[20WRX04]

IIRC, bank one is the front one. Bank 2 being the rear one.

[PRIMAJIN]

awsome that is a huge help bra, do u happen to know the part # for the front sensor?

[EJ257]

do u happen to know the part # for the front sensor?

22641aa211

[SD_GR]

i drive an 04 sti, the code read wen the CEL came on was
P0171 system too lean bank 1.

There is only one bank. "Bank 1" is simply a default.

Out of the two o2 sensors on the car, how do i find out witch one needs replacing?

Don't. Instead:

Clean your MAF.

If you have an aftermarket air intake, fix it by fitting the factory unit. Check for vacuum leaks and fix them.

Check the hose to the fuel pressure regulator.

Check for leaks on the boost side too -- a BOV, incidentally, is a considered a leak -- and fix them.

In all the instances of a PO171 I've seen, heard of, or read about, I think maybe two (if not just one) turned out to actually be the O2 sensor.

Search PO171 for troubleshooting tips.

For example:
P0171 CEL Thread - Everything you ever wanted to know? - NASIOC

If it turns out you cannot ID the problem, only then would I beg/borrow/steal a spare O2 sensor, fit it, and see if the CEL goes away. If it does not, go back to Step 1 and forget about the sensor (since that's not the issue).

[20WRX04]

^^^ +1 on what SD_GR said.