P1639

Circuit Description
The PCM uses the 5 Volt Reference B circuit as a sensor feed for the Air Conditioning (A/C) Refrigerant Pressure Sensor. The PCM monitors the voltage on the 5 Volt Reference B circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.

Conditions for Running the DTC
The ignition is on.

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

Action Taken When the DTC Sets
^ The PCM will illuminate the 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 Fail Records data.

Conditions for Clearing the MIL/DTC
^ The PCM will turn the MIL OFF 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 a scan tool.

Diagnostic Aids

Important: Inspect the 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 A/C High Side Pressure display on the scan tool while moving connectors and wiring harnesses related to the A/C refrigerant pressure sensor. A change in the A/C High Side Pressure display will indicate 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
The numbers below refer to the step numbers on the Diagnostic Table.
5. This step is checking for a short to voltage on the 5 volt ref line.
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.