Conditions for Setting the DTC: P0121
WARNING: This page is about a different car, the 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
The TP sensor 1 disagrees greater than 9 percent from TP sensor 2, or TP sensor 1 disagrees greater than 9 percent from the predicted value for greater than 2 seconds.