P1639

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Schematic:

CIRCUIT DESCRIPTION
The PCM uses the 5 volt Reference B circuit to feed the fuel tank pressure sensor.

The PCM monitors the voltage on the 5 volt Reference B circuit. If the voltage is outside an acceptable range, DTC P1639 will set.

CONDITIONS FOR RUNNING THE DTC
The ignition switch is ON.

CONDITIONS FOR SETTING THE DTC
^ The PCM detects a voltage out of range condition on the 5 volt Reference B circuit.
^ The condition exists for less than 1 second.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) during the second consecutive trip in which the diagnostic test runs and fails.
^ The PCM stores the conditions 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 after the third consecutive trip in which the diagnostic runs and passes.
^ 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 Clear DTC Information function.

DIAGNOSTIC AIDS
If the condition is intermittent, refer to Intermittent Conditions in Symptoms. Intermittent Conditions

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. If the scan tool displays 5 volt reference B fault the 5.0 volt reference circuit is either shorted to ground or shorted to a voltage.
3. This step determines if the fuel pump control circuit is causing the condition.
4. Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and/or Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate the vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This will isolate when the DTC failed.
5. This step determines the location of the condition. If the scan tool displays 5 volt reference B OK, then the fault is isolated to the fuel tank pressure sensor, or jumper harness.
6. This step determines if the 5 volt reference circuit is shorted to a ground.
7. If the DMM displays 5 volts, inspect the fuel tank jumper harness for signs of damage.