Conditions for Setting the DTC
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.
DTC P0787 sets if the following condition occurs for 5 seconds:
The TCM commands the solenoid OFF and the voltage feedback remains low, 0 volts.