Check engine light

I have a 2002 Trans Am WS6 automatic with 60000 miles on it. I just replaced the mass flow sensor and put a k&N airfilter on it but thought after that, that the car felt a little weaker but still strong from idle, and at about 60 mph if I floored it, it missed like it hesitated and then came pretty strong, and aftera bout 150 miles the check engine light came on, and I have the code checked and here is what it said, code number PO171.
Definition: fuel trim bank one condition
Explenation: The powertrain control module uses the oxygensensor to calculate the air/fuel ratio of the engine. The computer has recognized a rich or lean condition on one engine bank only.
Probable causes: 1 If bank one and two codes set together suspect fuel pressure or mass air flow sensor.
2. Oxygen sensor defective
3. Ignition missfire-repair
4. Fuel injector problem
trouble shooting: the pcm has determined that during testing, the fuel system for bank 1 was to lean. (Bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)
I had a new mass air flow sensor from granatelli that are supposed to flow more air and give more power but instead I got less power. But I have just put the stock mass air flow sensor back but the service engine light is still on. What do u think is the problem is it the mass airflow sensor that messed it up or oxygensensor or something else?