Subaru throwing P0171. Tailhappy, get in here...

So my mom's WRX is throwing a P0171 code, which reads something like it's running lean.  I thought it was probably the o2 sensor.  I did some Google'ing and it looks like it could be a small number of things. 

Anyway, looks like the easiest/free thing to do is clean out the MAF, so I'll do that. I doubt that's going to be it so next is the 02 sensor. The car is an 03 so one of them is probably due to die.  Anyway, if I just go to Autozone and get an OEM replacement Denso/Bosch, is there any that is better than any other?

I am thinking something like this one:

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_oxygen-sensor-direct-fit-denso_20471856-p?navigationPath=L1*14932%7CL2*16465 

My RSX's o2 sensor went bad a few months ago. To buy an OEM one was $300.  In a pinch I bought a Denso from Advanced Auto for $150 (yea, I know I over paid but I needed it that day) and it's been fine.

Do the free/easy/cheap stuff first, go with an OE sensor, and if needs a cat, its time to swap in a bigger turbo and a catless DP. Go big or go home.

How much is an OE sensor? The cat is $1200.

Not "going big" on my mom's car.

Haha, I think he was joking. It's pretty cool your mom drives a rex though

OE meaning a Denso or whatever from Napa/Carquest. They're usually cheaper, and its the same part, minus the Subaru sticker on the box.