Pulled this code on my 97 ssei bonneville. Does anyone know if this tends to be a pcm problem or a bad sensor causing the low voltage?

I would check the ground for the PCM, make sure it is clean and making good contact. Also check the wiring harness and ensure no wires have been pinched or cut, that could cause a short or low voltage situation. Otherwise that code does indicate an internal problem with the PCM.

DTC P1635 5 Volt Reference Circuit

Refer to A/C Presure, Fuel Tank Pressure, TP, MAP, IAT, ECT Sensors .
<A href="http://127.0.0.1:9001/servlets/BlobShtml?ShtmlFile=47368&evc=sm&pubid=43&cellId=2 5084&mspsdsubkey=362#ss1-12485">Circuit Description

The PCM uses the 5 Volt Reference A circuit as a sensor feed to the following sensors:

• The TP sensor.
• The MAP sensor.
• The EGR Valve Pintle Position sensor.

The PCM monitors the voltage on the 5 Volt Reference A circuit. If the voltage is out of tolerance, the PCM will set DTC P1635.
<A href="http://127.0.0.1:9001/servlets/BlobShtml?ShtmlFile=47368&evc=sm&pubid=43&cellId=2 5084&mspsdsubkey=362#ss1-45501">Conditions for Setting the DTC

• The PCM detects a voltage out of tolerance condition on the 5 Volt Reference A circuit.
• The above condition is present for longer than 19 seconds.

<A href="http://127.0.0.1:9001/servlets/BlobShtml?ShtmlFile=47368&evc=sm&pubid=43&cellId=2 5084&mspsdsubkey=362#ss1-12110">Action Taken When the DTC Sets

• The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
• The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

<A href="http://127.0.0.1:9001/servlets/BlobShtml?ShtmlFile=47368&evc=sm&pubid=43&cellId=2 5084&mspsdsubkey=362#ss1-12114">Conditions for Clearing the MIL/DTC

• The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
• The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
• The DTC can be cleared by using the scan tool.

<A href="http://127.0.0.1:9001/servlets/BlobShtml?ShtmlFile=47368&evc=sm&pubid=43&cellId=2 5084&mspsdsubkey=362#ss1-45513">Diagnostic Aids


Important:

Be sure to inspect PCM and engine grounds for being secure and clean.

Check for the following conditions:

• Poor connection at the PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
• Damaged harness. Inspect the wiring harness for damage.
• If the harness appears to be OK, observe the MAP display on the scan tool with the ignition ON, engine not running while moving connectors and wiring harnesses related to the following sensors. A change in the MAP display will indicate the location of the malfunction.
  • The TP sensor.
  • The MAP sensor.
  • The EGR valve.
  • The fuel tank pressure sensor (VIN K only).

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Keep in mind we have nearly every OBD code listed here. https://www.gmforum.com/showthread.p...84#post1391414