P1639

Circuit Description
The powertrain control module (PCM) uses the 5 volt Reference B circuit as a sensor feed for the following sensors:
^ The A/C refrigerant pressure sensor.
^ The fuel tank pressure sensor.
The PCM monitors the voltage on the 5 volt Reference B circuit. If the voltage is out of tolerance, the PCM sets DTC P1639.

Conditions for Setting the DTC
^ The PCM detects a voltage out of tolerance condition on the 5 volt Reference B circuit.
^ Above condition is present for longer than 10 seconds.

Action Taken When the DTC Sets
The PCM stores the conditions which were present when the DTC set as Fail Records data only.

Conditions for Clearing the MIL/DTC
^ A history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC may be cleared using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids

Important: Verify that the PCM and engine grounds are secure and clean.

Inspect for the following conditions:
^ A faulty connection at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.
^ Inspect the wiring harness for damage. If the harness appears to be OK, observe the A/C pressure display on the scan tool while moving the connectors and wiring harnesses related to the A/C refrigerant pressure sensor. A change in the A/C pressure display indicates the location of the malfunction.
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.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
11. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.