For 1990-2009 cars

## DTC P0451 w/o ORVR

### Circuit Description

The DTC 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 the fuel tank pressure or vacuum. The fuel tank pressure sensor signal voltage to the powertrain control module (PCM) varies from a minimum of about 0.2 volts with pressure in the fuel tank to a maximum of more than 4 volts with low pressure in the fuel tank.

### Conditions for Running the DTC

• | The barometric pressure is more than 75 kPa.

• | The engine coolant temperature is -8 to +110°C (18F-230°F).

• | The intake air temperature is more than -8°C (+18°F).

• | The fuel tank level is 25-75 percent.

• | The fuel tank pressure is more than or less than the atmospheric pressure.

• | The engine has been running at idle for at least 2 minutes.

### Conditions for Setting the DTC

The change in the fuel tank pressure is less than a specified value after completion of EVAP system leak test.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after 3 consecutively passing trips without a fault present.

• | A history DTC clears after 40 consecutive warm-up cycles without a fault.

• | Perform the scan tool Clear DTC Information function in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | A skewed fuel tank pressure sensor may cause DTC 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 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 electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If a DTC P0451 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the 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 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. The fuel tank pressure sensor reading indicated on the scan tool will vary with the surrounding barometric pressure.

3. The FTP sensor voltage will vary with the surrounding barometric pressure.

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

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

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

7. This step checks for a stuck EVAP canister vent valve or a clogged inlet air hose at the EVAP canister vent valve .

8. This step performs an EVAP canister vent solenoid test. The EVAP canister vent valve 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).

9. This step performs a control circuit test on the EVAP canister vent valve . The EVAP canister vent valve 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).

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 DTC P0450, P0443, P0440, or P0455 set along with DTC 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 (FTP) sensor parameter on the scan tool.

Is the pressure within the specified range?

0.0-0.9 mm Hg (0.0-0.5 in H2O)

Go to Step 12

Go to Step 4

4 |

1. Remove the fuel tank. Refer to Fuel Tank Replacement .
2. Remove the FTP 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.

Was any blockage found?

--

Go to Step 21

Go to Step 5

5 |

1. Turn ON the ignition, leaving the engine OFF.
2. Disconnect the FTP sensor electrical connector.
3. Measure the voltage at the harness side of the FTP sensor terminal 3 with a digital multimeter (DMM).

Does the voltage equal the specified value?

5 V

Go to Step 7

Go to Step 6

6 | Check for an open or a short in the 5-volt reference circuit between the FTP sensor and the PCM. Repair as necessary. Was a repair necessary? | -- | Go to Step 29 | Go to Step 25
7 | Probe terminal 2 of the FTP sensor on the harness side with a test lamp connected to B+. Does 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 FTP sensor and the PCM.
2. Repair as necessary.

Was a repair necessary?

--

Go to Step 29

Go to Step 25

9 |

1. Connect the FTP sensor electrical connector.
2. Backprobe terminal 1 of the FTP sensor connector with a DMM connected to ground.
3. Measure the voltage with the DMM set to the DC scale.

Is the voltage within the specified range?

2.5-2.7 V

Go to Step 10

Go to Step 26

10 | Apply a small amount of vacuum to the FTP sensor while observing the voltage on the DMM. Does the voltage drop when the vacuum is applied? | -- | Go to Step 11 | Go to Step 27
11 |

1. Check for an open in the FTP sensor signal circuit between the FTP sensor and the PCM.
2. Repair as necessary.

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 FTP 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 FTP 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 J 23738-A Vacuum Pump.
3. With the vacuum applied, blow GENTLY through the tank side port. Air should flow easily through to the canister port.

Does the FTP 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 valve.
3. Apply the specified value of vacuum to the canister side of the EVAP canister vent valve with the J 23738-A .

Does the valve hold the 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 valve ON.
3. Apply the specified value of vacuum to the canister side of the EVAP canister vent valve with the J 23738-A .

Does the valve hold the vacuum?

15 in Hg

Go to Step 16

Go to Step 17

16 | While the EVAP canister vent valve is still holding the vacuum from Step 15, use the scan tool to command the EVAP canister vent valve OFF. Does the vacuum decrease to the specified value? | 0.0 in Hg | Go to Diagnostic Aids | Go to Step 24
17 |

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

Does 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 valve ON.
2. Probe the harness side of terminal 2 of the EVAP canister vent valve with a test lamp connected to B+.

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 of EVAP canister vent valve 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 control circuit of the EVAP canister vent valve between the EVAP canister vent valve 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 passage. Is the action complete? | -- | Go to Step 29 | --
22 | Inspect the vacuum hoses to the EVAP canister vent valve for restrictions and repair as necessary. Was a repair necessary? | -- | Go to Step 29 | Go to Step 23
23 | Replace the EVAP canister vent valve. 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 control circuit of the EVAP canister vent valve between the EVAP canister vent valve 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 . Did you complete the replacement? | -- | Go to Step 29 | --
26 |

1. Check for a short in the fuel tank pressure sensor signal circuit between the FTP 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 FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Is the action complete? | -- | Go to Step 29 | --
28 | Replace the FTP 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.

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

## DTC P0451 w/ORVR

### Circuit Description

The DTC 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 the fuel tank pressure or vacuum. The fuel tank pressure sensor signal voltage to the powertrain control module (PCM) varies from a minimum of about 0.2 volts with pressure in the fuel tank to a maximum of more than 4 volts with low pressure in the fuel tank.

### Conditions for Running the DTC

• | The barometric pressure is more than 75 kPa.

• | The engine coolant temperature is -8 to +110°C (18-230°F).

• | The intake air temperature is -14 to +70°C (7-158°F).

• | The fuel tank level is 25-75 percent.

• | The fuel tank pressure is more than or less than the atmospheric pressure.

• | The engine has been running at idle for at least 2 minutes.

### Conditions for Setting the DTC

The fuel tank pressure is less than -40 mm Hg (-21.41 in H2O) or more than 40 mm Hg (21.41 in H2O) for 5 seconds.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after 3 consecutively passing trips without a fault present.

• | A history DTC clears after 40 consecutive warm-up cycles without a fault.

• | Perform the scan tool Clear DTC Information function in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | A skewed fuel tank pressure sensor may cause DTC 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 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 electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If a DTC P0451 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the 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 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. The fuel tank pressure sensor reading indicated on the scan tool will vary with the surrounding barometric pressure.

3. The FTP sensor voltage will vary with the surrounding barometric pressure.

4. This step verifies that the fuel tank pressure sensor is able to react to EVAP system pressure changes.

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

6. Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0451 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0451 appears in the Not Ran Since Code Cleared list, the DTC P0451 diagnostic has not yet run. When the DTC P0451 does not appear in the Not Ran Since Code Cleared list, the DTC P0451 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0451 diagnostic has passed. The 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 |

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 (FTP) sensor parameter on the scan tool.

Is the pressure within the specified range?

0.0-0.9 mm Hg (0.0-0.5 in H2O)

Go to Step 11

Go to Step 3

3 |

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.

Was any problem found?

--

Go to Step 13

Go to Step 4

4 |

1. Turn ON the ignition, leaving the engine OFF.
2. Disconnect the FTP sensor electrical connector.
3. Measure the voltage on the harness side of terminal 3 of the FTP sensor connector using a digital multimeter (DMM).

Does the voltage equal the specified value?

5 V

Go to Step 6

Go to Step 5

5 |

1. Check for an open in the 5-volt reference circuit between the FTP sensor and the PCM.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 18

Go to Step 14

6 | Probe the terminal 2 of the FTP sensor connector on the harness side using a test lamp connected to B+. Does the test lamp illuminate? | -- | Go to Step 8 | Go to Step 7
7 |

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

Was a repair necessary?

--

Go to Step 18

Go to Step 14

8 |

1. Connect the FTP sensor electrical connector.
2. Backprobe the connector terminal 1 with the positive lead and connect the negative lead to ground using the DMM.
3. Measure the voltage with the DMM set to the DC scale.

Is the voltage within the specified range?

2.5-2.7 V

Go to Step 9

Go to Step 15

9 | Apply a small amount of vacuum to the FTP sensor while observing the DMM. Does the voltage decrease when the vacuum is applied? | -- | Go to Step 10 | Go to Step 16
10 |

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

Was a repair necessary?

--

Go to Step 18

Go to Step 14

11 | 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 (FTP) control valve. Refer to Fuel Tank Pressure Control Valve Replacement .
2. Connect a vacuum hose to the canister side port of the FTP control valve.

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

3. Blow air into the hose on the FTP control valve.

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

--

Go to Step 12

Go to Step 17

12 | Important: The FTP control valve allows vapor to flow from fuel tank to the canister when the fuel tank pressure is greater than canister pressure by more than 3.5 kPa (0.5 psi).

1. Connect a hose to the fuel tank port of the FTP control valve.
2. From the tank side port, blow hard through the valve. Air should flow through the valve out of the canister port.

Does the FTP control valve pass this test?

--

Go to Diagnostic Aids

Go to Step 17

13 | Clean the sensor orifice. Is the action complete? | -- | Go to Step 18 | --
14 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 18 | --
15 |

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

Was a repair necessary?

--

Go to Step 18

Go to Step 16

16 | Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Is the action complete? | -- | Go to Step 18 | --
17 | Replace the FTP control valve. Refer to Fuel Tank Pressure Control Valve Replacement . Is the action complete? | -- | Go to Step 18 | --
18 |

1. Turn OFF the ignition.
2. Remove the DOME fuse for 30 seconds.
3. Install the DOME fuse.
4. Reset the radio clock time. Refer to Radio/Audio System Description and Operation in Entertainment.
5. Start the engine.
6. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or until the DTC P0451 diagnostic test has run.

Does the DTC run and pass?

--

Go to Step 19

Go to Step 2

19 | 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