DTC P1635: 5-Volt Reference Circuit: 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.
The 5-volt reference circuit provides a signal voltage to the EGR valve position sensor, fuel tank pressure sensor, MAP sensor and TP sensor. The current for each 5-volt reference circuit flows through the sensor ground in the PCM. The PCM regulates the 5-volt reference voltage under all normal operating conditions. PCM monitors 5-volt reference circuit for low and high voltage faults. DTC sets when 5-volt reference circuit is outside a calibrated voltage range.