2006 Pontiac G6 GTP. Engine Size 3.9 V6.
Question: Check Engine light on. Code reads PO121. Took it to a shop to have it put of the diagnostic machine to tell I was told it was the air filter and the air flow had some dirt. They said the fixed it but I am having the same problems and the check engine light is on again.
Answer: Code P0121 is for the throttle position sensor performance. An extremely plugged air filter can cause this problem. Since this is not the case with your Pontiac, I would suspect an electrical problem such as a poor connection at the throttle body, carbon buildup in the throttle or a faulty throttle body itself. Other possibilities are a vacuum leak, or cracked intake air duct or a disconnected vent tube on the rear valve cover under the coil / ignition module assembly. A faulty MAP sensor can also cause this problem, but should set a code for it, like P0106, P0007 or P0108. A faulty MAF sensor will also cause this, but usually has a code P0101 or P1101 as well.