M54 running lean question

[rwm514]

Hey guys,
I recently talked my GF into buying a 2006 X3 3.0i. Unfortunately I couldn't get any service history but the car is in really nice shape, bought it in phoenix.
First day she drove it on the freeway up hill it was hesitating a bit and finally threw a SES. I just picked up a foxwell 520 and it read "o2 sensor precat bank one".
I ordered 2 new bosch o2 sensors ($105 a piece) and just got them last night.
In the meantime I cleared the code and driving it around town she got another SES. I figured it would be the same but this time 2882 and 2883. Which is "mixture prep bank 1 and 2"
She also has been getting the "check gas cap" from time to time. I'm wondering if it might be the fuel pump.
Any one have any experience with this? I'm thinking just install the new o2 sensors and go from there. Thanks