P1639

CIRCUIT DESCRIPTION
The powertrain control module (PCM) uses a common 5-volt reference circuit as a sensor feed to the fuel tank pressure (FTP) sensor and the throttle position (TP) sensor.

The PCM monitors the voltage on the 5-volt reference circuit. This diagnostic trouble code (DTC) sets if the voltage is out of range.

CONDITIONS FOR RUNNING THE DTC
The engine is running.

CONDITIONS FOR SETTING THE DTC
The 5-volt reference circuit is out of range for more than 2 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.

DIAGNOSTIC AIDS
^ If you cannot duplicate the DTC, use the Freeze Frame/Failure Records in order to locate an intermittent condition in the following ways:
- Use the information included in the Freeze Frame/Failure Records in order to determine how many miles since the DTC set.
- Use the Fail Counter and Pass Counter in order to determine how many ignition cycles the diagnostic test reported as a pass or a fail.
- Operate the vehicle within the same Freeze Frame conditions that you observed in order to isolate when the DTC failed. Use the following conditions for your tests:
^ The RPM
^ The load
^ The vehicle speed
^ The temperature
^ For an intermittent condition, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-3:

Steps 4-11:

The numbers below refer to the step numbers on the diagnostic table.
2. The 5-volt reference circuits for the sensors are connected together inside the PCM. If both DTCs P1635 and P1639 set at the same time, a 5-volt reference circuit is shorted to a voltage.
3. The 5-volt reference circuits for the sensors are connected together inside the PCM. When only P1639 is set, this indicates a short to ground on one of the 5-volt reference circuits.
5. This step determines if a 5-volt reference circuit is shorted to ground.
6. The 5-volt reference circuits may be shorted to another PCM circuit. The shorted circuit may not be apparent when the PCM harness connector is disconnected. Testing the continuity from each 5-volt reference circuit isolates the shorted circuit.