Conditions for Setting the DTC: P0990
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
the TCM detects that the TFP Switch 5 signal voltage is high, 12 volts when the low and reverse clutch or the 4-5-6 clutch is commanded OFF, then the TCM sets DTC P0990.