Circuit Description
The fuel tank pressure sensor changes resistance based on the fuel tank pressure, or vacuum. A vacuum is considered negative pressure.
The powertrain control module (PCM) provides a 5 volt reference and a ground. The fuel tank pressure sensor signal varies between ground and 5 volts as the fuel tank pressure varies. The fuel tank pressure sensor voltage increases as the fuel tank pressure decreases, such as negative pressure, or vacuum. The fuel tank pressure sensor voltage decreases, such as positive pressure, or low voltage, as the fuel tank pressure increases. The PCM uses the fuel tank pressure sensor voltage for evaporative emission (EVAP) system leak detection. This DTC sets when the fuel tank pressure sensor signal goes below a predetermined value.
Conditions for Running the DTC
The ignition switch is in the RUN position.
Conditions for Setting the DTC
| • | The fuel tank pressure sensor voltage is less than 0.1 volts. |
| • | All conditions are present for more than 5 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL and the DTC. |
Diagnostic Aids
Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.
For an intermittent, refer to Symptoms .
The EVAP pressure sensor range is from +7.5 in. H2O of pressure to -17.5 in. H2O of pressure or vacuum.
Inspect for poor connections at connector C152. This connector is located under the underhood electrical center. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
The vacuum and pressure in the EVAP system is measured in inches of water (H2O). Most gauges measure vacuum in inches of mercury (Hg) and pressure in pounds per square inch (psi). The following shows how the values compare:
in H2O | in Hg | psi |
|---|---|---|
1in H2O | 0.07 in Hg | 0.0361 psi |
5 in H2O | 0.36 in Hg | 0.180 psi |
10 in H2O | 0.73 in Hg | 0.361 psi |
15 in H2O | 1.10 in Hg | 0.541 psi |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step determines if the condition is present.
-
Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and 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 or a fail. Operate the vehicle within the same Freeze Frame conditions, such as RPM, load, vehicle speed, temperature etc., that you observed. This will isolate when the DTC failed.
-
If the scan tool displays 5.0 volts, the fuel tank pressure sensor signal circuit and the PCM are OK.
-
This step determines if the signal circuit is open.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Did DTC P1639 fail this ignition? | -- | |||
|
Important: The ignition must be OFF for a minimum of 15 seconds when you are instructed to turn the ignition OFF. If the ignition switch is not in the OFF position, mis-diagnosis occurs. This information applies to every step that instructs you to turn OFF the ignition.
Is the fuel tank pressure sensor voltage below the specified value? | 0.10 V | |||
Does the scan tool indicate that this diagnostic failed this ignition? | -- | Go to Diagnostic Aids | ||
Is the fuel tank pressure sensor voltage at the specified value? | 5 V | |||
Is the fuel tank pressure sensor voltage near the specified value? | 5 V | |||
7 |
Is the fuel tank pressure sensor signal circuit open or shorted to ground? | -- | ||
8 |
Did you find the 5 volt reference circuit open? | -- | ||
9 | Repair the open 5 volt reference circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
10 | Repair the open or grounded fuel tank pressure sensor signal circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
11 |
Did you find and correct the condition? | -- | ||
12 |
Did you find and correct the condition? | -- | ||
13 |
Important: Inspect for poor connections at the fuel tank pressure sensor before replacing the sensor. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Replace the fuel tank pressure sensor. Refer to Fuel Tank Pressure Sensor Replacement . Is the action complete? | -- | -- | |
14 |
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | -- | |
15 |
Does the scan tool indicate that this test ran and passed? | -- | ||
16 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
