The powertrain control module (PCM) monitors the fuel tank pressure (FTP) sensor signal in order to detect vacuum decay and excess vacuum during the enhanced evaporative emission (EVAP) diagnostic. The PCM supplies a 5-volt reference and ground to the sensor. The FTP sensor signal voltage increases as the fuel tank pressure decreases (negative pressure or vacuum, high voltage). The FTP sensor signal voltage decreases as the fuel tank pressure increases (positive pressure, low voltage). This DTC sets when the FTP sensor signal goes below a predetermined value.
The engine is running
• | The fuel tank pressure sensor voltage is less than 0.1 volts |
• | All conditions present for greater than 5 seconds |
• | 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. |
• | 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. |
The numbers below refer to the step numbers on the diagnostic table.
If DTC P1639 sets, the 5-volt reference may be shorted to ground.
If the FTP sensor voltage is near 5 volts, the FTP sensor signal and 5-volt reference circuits are OK from the fuel tank harness connector to the PCM.
Action | Values | Yes | No | |||||||
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Schematic Reference:
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1 | Did you perform the Diagnostic System Check-Engine Controls ? | -- | Go to Step 2 | |||||||
Did DTC P1639 fail this ignition? | -- | Go to DTC P1639 | Go to Step 3 | |||||||
3 | With the scan tool, observe the fuel tank pressure (FTP) sensor voltage. Does the scan tool indicate that the FTP sensor voltage is less than the specified value? | 0.1 volts | Go to Step 5 | Go to Step 4 | ||||||
4 |
Does the DTC fail this ignition? | -- | Go to Step 5 | Go to Intermittent Conditions | ||||||
Does the scan tool indicate that the FTP sensor voltage is near the specified value? | 5.0 volts | Go to Step 8 | Go to Step 6 | |||||||
6 | Test the 5 volt reference circuit of the FTP sensor for an open, between the fuel tank harness connector and the PCM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 7 | ||||||
7 | Test the signal circuit of the FTP sensor for a short to ground, or an open between the fuel tank harness connector and the PCM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 9 | ||||||
8 |
Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 10 | ||||||
9 | Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 11 | ||||||
10 | Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Did you complete the replacement? | -- | Go to Step 12 | -- | ||||||
11 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 12 | -- | ||||||
12 |
Does the DTC run and pass? | -- | Go to Step 13 | Go to Step 2 | ||||||
13 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that have not been diagnosed? | -- | System OK |