DTC P1603: TCM Trouble Code Detected: DTC Detecting Condition
WARNING: This page is about a different car, the 2006 Suzuki Grand Vitara. However, it is still accessible from the selected car via links, so may be relevant.
When ECM receives a trouble code from TMC, which indicates that some problem occurred in sensor circuits and its calculated values used for operations such as idle speed control, engine power control, and so on by TCM, ECM sets DTC P1603. (TCM outputs the trouble code to ECM when TCM can not compute the engine control signal due to malfunctions of sensor circuits used for gear shift control).