DTC P1634: Improper Ignition 1 Input: Description
WARNING: This page is about a different car, the 2003 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
The Powertrain Control Module (PCM) receives 3 ignition inputs from the ignition switch: ignition 0 signal, ignition 1 signal, and crank signal. The PCM receives the ignition 1 signal with the ignition switch in the ON and START positions. The PCM uses a Class II ignition mode message and voltage input from the ignition switch to test the ignition 1 voltage circuit. If the PCM detects an improper ignition 1 signal, DTC P1634 will set.