For 1990-2009 cars

### Circuit Description

The DCT P0451 Fuel Tank Pressure (FTP) Sensor Performance diagnostic checks the accuracy of the FTP sensor signal. The fuel tank pressure sensor responds to changes in fuel tank pressure or vacuum. The fuel tank pressure sensor signal voltage to the PCM varies from a minimum of about 0.2 volts with pressure in the fuel tank to above 4 volts with a high vacuum situation in the fuel tank (vent solenoid stuck closed).

### Conditions for Running the DTC

• | Engine coolant temperature between 70°C (158°F) and 110°C (230°F).

• | Intake air temperature between -10°C (+14°F) and +50°C (+122°F).

• | Barometric pressure more than 75 kPa.

• | Fuel tank level is between 25 and 75 percent.

### Conditions for Setting the DTC

• | Engine running at idle for at least 2 minutes.

• | Fuel tank pressure is lower than specified value after completion of EVAP system leakage check.

• | The EGR system diagnostic has run and passed.

### Action Taken When the DTC Sets

• | The malfunction indicator lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL will turn OFF after three consecutively passing cycles without a fault present.

• | A History DTC will clear after 40 consecutive warm-up cycles without a fault.

• | DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.

### Diagnostic Aids

Check for any of the following conditions:

• | A shifted (incorrectly calibrated) fuel tank pressure sensor may cause a P0451 to set. Compare voltage readings with a known good sensor.

• | A plugged sensor air vent hole -- Inspect the sensor and clean if necessary.

• | If multiple DTCs are set, it may be possible to have an open or shorted reference voltage circuit or an open ground circuit.

If a DTC P0451 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance Evaporative Emission System Set .

An intermittent malfunction may be caused by a fault in the fuel tank pressure sensor circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If you cannot duplicate a DTC P0451, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

### Test Description

The numbers below refer to the step numbers in the diagnostic table.

1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and to store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step determines if a fault is present. Review the Freeze Frame data in order to determine the condition of when the DTC set. Always record this information.

3. This step verifies that the fuel tank pressure sensor accurately reacts to EVAP system pressure changes. This also checks the sensor signal input circuit.

4. This step checks the operation of the fuel tank pressure control valve.

5. This step checks the operation of the fuel tank pressure control valve.

6. This step checks for a stuck EVAP canister vent solenoid or a clogged EVAP canister vent solenoid inlet air hose.

7. This step performs an EVAP canister vent solenoid test. The EVAP canister vent solenoid output control is disabled by the PCM when any DTCs are stored, when engine speed is present or when the engine operating temperature is less than 80°C (176°F).

8. This step performs an EVAP canister vent solenoid control circuit test. The EVAP canister vent solenoid output control is disabled by the PCM when any DTCs are stored, when engine speed is present, and when the engine operating temperature is less than 80°C (176°F).

9. This step checks whether the conditions that caused the DTC P0451 are still present. Driving the vehicle under the following conditions can verify whether the fault is present. After performing the scan tool Clear DTC Information function, road test the vehicle while monitoring the DTC P0451 diagnostic on the scan tool under Not Ran Since Code Cleared. If a DTC P0451 appears in the Not Ran Since Code Cleared list, the P0451 diagnostic has not yet run. When the DTC P0451 does not appear in the Not Ran Since Code Cleared list, the P0451 diagnostic has run. If the MIL is NOT ILLUMINATED and there is no PENDING DTC Status in DTC Information, the P0451 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Did a DTC P0450, P0443, P0440, or P0455 set along with the P0451? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Step 3
3 |

1. Turn OFF the ignition.
2. Remove the fuel filler cap.
3. Turn ON the ignition, leaving the engine OFF.
4. Select the fuel tank pressure sensor parameter on the scan tool.

Does the scan tool display the specified value?

0.0-0.9 mm Hg (0.0-0.5 in Hg)

Go to Step 12

Go to Step 4

4 |

1. Remove the fuel tank. Refer to Fuel Tank Replacement .
2. Remove the fuel tank pressure sensor from the fuel tank. Refer to Fuel Tank Pressure Sensor Replacement .
3. Visually inspect the sensor air vent hole and the air pressure inlet passage for clogging.

Did you find any blockage?

--

Go to Step 21

Go to Step 5

5 |

1. Turn ON the ignition, leaving the engine OFF.
2. Disconnect the fuel tank pressure sensor electrical connector.
3. Measure the voltage at the fuel tank pressure sensor terminal 3 (harness side) with a DMM.

Is the voltage at the specified value?

5 V

Go to Step 7

Go to Step 6

6 |

1. Check for an open or a short in the 5 volt reference circuit between the fuel tank pressure sensor and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 29

Go to Step 25

7 | Probe terminal 2 of the fuel tank pressure sensor, harness side, with a test lamp connected to B+. Did the test lamp illuminate? | -- | Go to Step 9 | Go to Step 8
8 |

1. Check for an open in the sensor ground circuit between the fuel tank pressure sensor and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 29

Go to Step 25

9 |

1. Reconnect the fuel tank pressure sensor electrical connector.
2. Backprobe the fuel tank pressure sensor connector terminal 1 with a DMM to ground.
3. Measure the voltage with the DMM set to the DC scale and compare the voltage to the specified value.

Was the voltage measured within the specified value?

2.0-2.5 V

Go to Step 10

Go to Step 26

10 | Apply a small amount of vacuum to the fuel tank pressure sensor while observing the voltage on the DMM. Did the voltage drop when the vacuum was applied? | -- | Go to Step 11 | Go to Step 27
11 |

1. Check for an open in the fuel tank pressure sensor signal circuit between the fuel tank pressure sensor and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 29

Go to Step 25

12 | Caution: Do not breathe the air through the EVAP component tubes or hoses. The fuel vapors inside the EVAP components may cause personal injury.

1. Remove the fuel tank pressure control valve. Refer to Fuel Tank Pressure Control Valve Replacement .

Important: There will be some resistance, but air should flow steadily through the valve.

2. From the canister port, blow air through the valve.

Did air flow through the valve to the tank port side?

--

Go to Step 13

Go to Step 28

13 |

Important: The fuel tank pressure control valve will not allow air to flow from the tank side port to the canister side port until the pressure on the tank side is more than 3.5 kPa (0.5 psi).

1. From the tank side port, blow HARD through the valve. Air should flow through to the canister port.
2. Apply the specified value of vacuum to the vacuum port of the fuel tank pressure control valve with a hand held vacuum pump.
3. With the vacuum applied, blow GENTLY through the tank side port. Air should flow easily through to the canister port.

Did the fuel tank pressure control valve pass both checks?

15 in Hg

Go to Step 14

Go to Step 28

14 |

1. Turn OFF the ignition.
2. Remove the canister hose from the EVAP canister vent solenoid.
3. Apply the specified value of vacuum to the canister side of the EVAP canister vent solenoid with the hand vacuum pump.

Does the solenoid hold vacuum?

15 in Hg

Go to Step 22

Go to Step 15

15 |

1. Turn ON the ignition, leaving the engine OFF.
2. Use a scan tool in order to command the EVAP canister vent solenoid ON.
3. Apply the specified value of vacuum to the canister side of the EVAP canister vent solenoid with the hand vacuum pump.

Did the solenoid hold vacuum?

15 in Hg

Go to Step 16

Go to Step 17

16 | While the EVAP canister vent solenoid is still holding vacuum from Step 15, use the scan tool to command the EVAP canister vent solenoid OFF. Did the vacuum drop to the specified value? | 0 in Hg | Go to Diagnostic Aids | Go to Step 24
17 |

1. Turn OFF the ignition.
2. Disconnect the EVAP canister vent solenoid electrical connector.
3. Turn ON the ignition, leaving the engine OFF.
4. Probe terminal 1, harness side, of the EVAP canister vent solenoid with a test lamp connected to ground.

Did the test lamp illuminate?

--

Go to Step 18

Go to Step 19

18 |

1. Use a scan tool in order to command the EVAP canister vent solenoid ON.
2. Probe terminal 2, harness side, of the EVAP canister vent solenoid with a test lamp connected to battery positive.

Did the test lamp illuminate?

--

Go to Step 23

Go to Step 20

19 | Repair the open or the short in the BLU/BLK wire between terminal 1 and the main relay. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 29 | --
20 |

1. Check for an open in the EVAP canister vent solenoid control circuit between the EVAP canister vent solenoid and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 29

Go to Step 25

21 | Clean the sensor air passages. Is the action complete? | -- | Go to Step 29 | --
22 | Inspect the EVAP canister vent solenoid vacuum hoses for restrictions and repair as necessary. Was a repair necessary? | -- | Go to Step 29 | Go to Step 23
23 | Replace the EVAP canister vent solenoid. Refer to Evaporative Emission Canister Vent Solenoid Valve Replacement . Is the action complete? | -- | Go to Step 29 | --
24 |

1. Check for a short to ground in the EVAP canister vent solenoid control circuit between the EVAP canister vent solenoid and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 29

Go to Step 25

25 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the replacement complete? | -- | Go to Step 29 | --
26 |

1. Check for a short in the fuel tank pressure sensor signal circuit between the fuel tank pressure sensor and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 29

Go to Step 27

27 | Replace the fuel tank pressure sensor. Refer to Fuel Tank Pressure Sensor Replacement . Is the action complete? | -- | Go to Step 29 | --
28 | Replace the fuel tank pressure control valve. Refer to Fuel Tank Pressure Control Valve Replacement . Is the action complete? | -- | Go to Step 29 | --
29 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text. Refer to the Test Description.

Does the DTC run and pass?

--

Go to Step 30

Go to Step 2

30 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK