98 Taurus check engine code P1131.....HELP

hi i have a 98 taurus with 78k miles 3.0 vulcan, and the check engine light came on, i had it read at autozone and they came up with the code P1131. on the little ticket they printed me out it says

TROUBLECODE OBDII (ALPHA) P CODE
TROUBLESHOOTING P1131
lack of ho2s switch - sensor indicates lean

what does this mean? lean what?

im guessing its either an o2 sensor, or mass air flow sensor, but its only a guess,if so what do you think the price of the two items would be. thanks in advance

I don't think it is the MAF. If MAF were the culprit, you would have a code for both banks. Remove the suspect O2 and see if there is a lot of carbon built up on it. Usually, in our shop, lack of switching is a lazy sensor and needs replaced. The sensor is the first sensor on the side that has the #1 cylinder on it.(rear next to firewall)

the car acts like its going to stall sometimes, the rpms get real low, and also one of the other thing is the car accelerates with a one or two second delay. any thoughts?

how do i know which o2 sensor is the culprit?

The sensor is the first sensor on the side that has the #1 cylinder on it.(rear next to firewall) This is the one. It is in front of the converter.