The PCM uses the 5-volt reference 2 circuit as a sensor feed for the fuel tank pressure (FTP) sensor. The PCM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is out of tolerance, the PCM will set DTC P1639.
The engine must be running.
• | The PCM detects a voltage out of tolerance condition on the 5-volt reference 2 circuit. |
• | Above condition is present for longer than 10 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. |
Number below refers to the step number on the Diagnostic Table.
Step | Action | Values | Yes | No |
---|---|---|---|---|
Schematic Reference:
| ||||
1 | Did you complete the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
2 |
Does the DTC fail this ignition? | -- | Go to Step 3 | Go to Intermittent Conditions |
3 |
Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 4 |
4 |
Does the voltage measure near the specified value? | 5 V | Go to Step 11 | Go to Step 5 |
5 | Does the voltage measure more than the specified value? | 5 V | Go to Step 6 | Go to Step 7 |
6 |
Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12 |
Important: The Ignition must me turned OFF for this test.
Does the resistance measure at the specified value? | ∞ or OL | Go to Step 8 | Go to Step 9 | |
8 |
Does the resistance measure at the specified value? | ∞ or OL | Go to Step 12 | Go to Step 9 |
9 | Repair the circuit for a short to chassis ground or a short to FTP sensor low reference. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 13 | -- |
10 | Test the signal circuit of the FTP sensor for a short to voltage and repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct a condition? | -- | Go to Step 13 | Go to Step 11 |
11 | Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- |
12 | Replace the PCM. Refer to Powertrain Control Module Replacement Did you complete the replacement? | -- | Go to Step 13 | -- |
13 |
Does the DTC run and pass? | -- | Go to Step 14 | Go to Step 2 |
14 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |