DTC P1133: O2S1 Circuit - Insufficient Switching: Description
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2001 Saturn SW2, 2001 Saturn SL2, 2001 Saturn SL1, 2001 Saturn SL, and 2001 Saturn SC2. However, it is still accessible from the selected car via links, so may be relevant.
Oxygen sensor responds to oxygen content in exhaust manifold. Sensor voltage is read by the PCM to determine a rich/lean O2S1 signal used to adjust injector pulse width. Normal sensor reading will fluctuate between 10-999 millivolts. DTC sets when O2S1 signal has too few switches over a certain length of time.