For 1990-2009 cars

### Circuit Description

The DTC P0451 Fuel Tank Pressure (FTP) Sensor Performance diagnostic checks the accuracy of the FTP sensor signal. Emission laws require that the powertrain control module (PCM) detect any leakage within the EVAP system. The PCM tests the EVAP system for leaks by monitoring the vapor pressures in the fuel tank with the FTP sensor. The FTP sensor contains a silicon based chip that changes electrical resistance when pressure is applied. The PCM supplies a 5-volt reference, an electrical ground, and a signal return circuit that has a 5-volt bias. The FTP sensor signal voltage varies from a minimum of about 0.2 volts with negative pressure in the fuel tank, to a maximum of more than 4 volts with positive pressure in the fuel tank.

Fuel Tank Vapor Space | FTP mm Hg | FTP Signal Voltage
--- | --- | ---
Pressure | Positive Value | High
No Pressure | Near Zero | 2.5-2.6 V
Vacuum | Negative Value | Low

### Conditions for Running the DTC

• | DTC P1450 and DTC P1451 are not set.

• | The barometric pressure (BARO) is more than 75 kPa.

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

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

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

• | The FTP 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 -40.0 mm Hg (-21.41 in H2O) or more than 40.0 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 the following conditions:

• | A skewed FTP sensor may cause DTC P0451 to set. Compare voltage readings with a known good sensor.

• | Inspect for obstructions and restrictions in the vapor passages that flow to the FTP sensor. Conditions that affect the pressure reaching the FTP sensor can cause a DTC P0451 to 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. The FTP 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 FTP sensor is able to react to evaporative emission (EVAP) system pressure changes.

5. This step checks for a blockage or a restriction that can trap pressure inside the fuel tank.

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 DTC P0451 appears in the Not Ran Since Code Cleared list, the P0451 diagnostic has not yet run. When 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. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. 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, with the engine OFF.
4. Select the 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 FTP sensor from the fuel tank. Refer to Fuel Tank Pressure Sensor Replacement .
3. Visually inspect the sensor orifice and the air pressure inlet passage for clogging.
4. Clean the FTP sensor orifice and inlet passage as necessary.

Did you find and correct a condition?

--

Go to Step 15

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Disconnect the FTP sensor electrical connector.
3. Turn ON the ignition, with the engine OFF.
4. Measure the voltage of the 5-volt reference circuit with a DMM connected to ground.

Is the voltage at the specified value?

5.0 V

Go to Step 6

Go to Step 5

5 |

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

Did you find and correct a condition?

--

Go to Step 15

Go to Step 12

6 | Connect a test lamp between the ground circuit of the FTP sensor and B+. Does the test lamp illuminate? | -- | Go to Step 8 | Go to Step 7
7 |

1. Test for an open or high resistance in the FTP sensor ground circuit.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 15

Go to Step 12

8 |

1. Turn OFF the ignition.
2. Connect the FTP sensor electrical connector.
3. Backprobe the FTP sensor input circuit at the PCM with a DMM connected to ground.
4. Turn ON the ignition, with the engine OFF.
5. Measure the voltage with the DMM set to the DC scale.

Is the voltage within the specified range?

2.5-2.6 V

Go to Step 9

Go to Step 10

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 14
10 |

1. Test the FTP sensor input circuit for the following conditions:

• | High resistance

• | Short to voltage

2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 15

Go to Step 14

11 |

1. Inspect the EVAP system for the following conditions:

• | Test the FTP control valve for proper operation. Refer to Evaporative Emission Control System Diagnosis .

• | Inspect for a stuck closed or blocked EVAP canister vent valve. Refer to Evaporative Emission Control System Diagnosis .

• | Inspect the vapor lines and hoses between the EVAP canister and the fuel tank for restrictions or blockage.

2. Repair or replace the component as necessary.

Did you find and correct a condition?

--

Go to Step 15

Go to Diagnostic Aids

12 |

1. Inspect the PCM electrical connectors for faulty terminal contact. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

13 | Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 15 | --
14 | Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Did you complete the replacement? | -- | Go to Step 15 | --
15 |

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 Freeze Frame conditions that set the DTC or until the P0451 diagnostic test has run. Refer to the test description.

Does the DTC run and pass?

--

Go to Step 16

Go to Step 2

16 | 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