Lean Machine

Recently purchased a 2000 Prizm from an independent dealer. Check engine light was on, so he replaced O2 sensor. Still getting check engine light and low speed stalling. Replaced fuel pump and filter. Still get the light, still stalls. De-carbonized the intake and upper engine. Still get the light. Codes state PO171 System Too Lean (Bank1). I am ready to return the car to the dealer under NY lemon law, but hoped someone might have seen this problem on 2000's and could suggest a fix (besides replacement of the engine!)

P.S. Over on the Toyota board, someone said that this code often results from a dirty or faulty MAF sensor. I can't get the torx screws that hold it in place off, but at least that's a lead I can pursue with the Chevy dealer and the indy that sold me the car. Does anyone have any experience with a faulty or dirty MAF sensor in a Prizm or Toyota Corolla?
Also mentioned were: dirty air filter (it's new), an loose or defective fuel cap (seems tight, but the MAF sensor is my first line of attack). Love to hear from someone who knows this problem.

It's still stalling. Might be the MAF.

You tell your dealer to clean the MAF sensor at his time and expense, if not, return the car. If he protests, inform him that this car will not pass NYS emmision and safety inspection as is. Tell the dealer you will now take down their inspection number and notify the DMV that they sold you a car with the MIL indicator on, and refuse to make it right