P1639

CIRCUIT DESCRIPTION
The engine control module (ECM) uses the 5-volt reference 2 circuit as a sensor feed to the following sensors:
^ The boost sensor
^ The accelerator pedal position (APP) sensor 2
^ The exhaust gas recirculation (EGR) vacuum sensor, if applicable.
^ The barometric pressure (BARO) sensor.

The ECM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is out of tolerance, the ECM will set DTC P1639.

CONDITIONS FOR RUNNING THE DTC
The ignition is ON.

CONDITIONS FOR SETTING THE DTC
^ The ECM detects a voltage out of tolerance condition on the 5-volt reference 2 circuit.
^ The above condition is present for longer than 10 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records.If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

TEST DESCRIPTION

DTC P1639 - Steps 1-7:

DTC P1639 - Steps 8-13:

The numbers below refer to the step numbers on the diagnostic table.
8. If the circuit is shorted to the transmission control module (TCM) MIL request circuit, the scan tool will show skewed BARO and BOOST sensor readings just after the ignition is turned ON. The sensor readings will then return to normal in under 1 second.
11. This vehicle is equipped with a ECM which utilizes an electrically erasable programmable read only memory (EEPROM). When the ECM is being replaced, the new ECM must be programmed.