Circuit Description
The DTC P0451 Fuel Tank Pressure (FTP) Sensor Performance diagnostic checks the accuracy of the FTP sensor signal. The evaporative emission (EVAP) control system prevents the emission of the fuel vapors. The fuel vapor generated in the fuel tank while driving or idling enters the EVAP canister where a charcoal element absorbs and stores the fuel vapor. When certain engine operating conditions are met the powertrain control module (PCM) purges the EVAP canister of stored vapors by turning ON the EVAP canister purge valve.
Emissions laws require that the PCM detect any leakage within the EVAP system. The PCM monitors the EVAP system for leaks by controlling and observing the vapor pressures in the EVAP system. The FTP sensor is used to detect any unexpected pressure in the EVAP system. The PCM supplies a 5-volt reference and an electrical ground to the FTP sensor. The FTP sensor contains a silicon based chip that changes electrical resistance when pressure is applied. The sensor converts the positive or the negative pressure into an electrical signal and sends the signal to the PCM.
Fuel Tank Vapor Space | FTP mmHg | FTP Signal Voltage |
|---|---|---|
Pressure | Positive Value | High |
No Pressure | Near Zero | 3.0-3.6V |
Vacuum | Negative Value | Low |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0451 Fuel Tank Pressure (FTP) Sensor Performance
Conditions for Running the DTC
Condition 1
| • | Intake air temperature is between 4-35°C (40-95°F). |
| • | Atmospheric pressure is between 70-110 kPa. |
| • | Battery voltage is more than 10.5 volts. |
| • | The engine is running or the ignition has been off continuously for at least 5 hours. |
| • | DTC P0451 runs once per drive cycle when the above conditions have been met. |
Condition 2
| • | Intake air temperature is between 4-35°C (40-95°F). |
| • | Atmospheric pressure is between 70-110 kPa. |
| • | Battery voltage is more than 10.5 volts. |
| • | The ignition has been off continuously for at least 5 hours. |
| • | DTC P0451 runs once per drive cycle when the above conditions have been met. |
Conditions for Setting the DTC
Condition 1
The FTP sensor output voltage rapidly shifts from inside to outside a specified range 10 times within 10 seconds.
Condition 2
The FTP sensor signal appears fixed during the FTP sensor test portion of the engine off EVAP diagnostic.
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 three consecutively passing trips without a fault present. |
| • | A History DTC clears after 40 consecutive warm-up cycles without a fault. |
| • | Use the scan tool Clear DTC Information function. |
Diagnostic Aids
Check for any of the following conditions:
| • | Inspect for a skewed or shifted FTP sensor. |
| • | The EVAP system diagnostic can be monitored on the scan tool under the I/M System Information selection. When the EVAP test displays a YES status, indicating that the system diagnostic is completed, check for DTC P0451 in the Last Test Failed screen of the scan tool. If there is no DTC P0451 displayed the EVAP system diagnostic has run and passed, indicating that no malfunction was present this time. DTCs MUST be cleared in order to view the Current Status of the system diagnostics being performed. Do not forget that the I/M System Information tests only indicate that the test has run, not if the test passed or failed. The Last Test Failed screen must be checked for related DTCs in order to determine the outcome of the diagnostic test involved. |
An intermittent malfunction may be caused by a fault in the FTP sensor electrical circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .
Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.
The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
The Diagnostic System Check - Vehicle 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.
-
This step determines if a FTP sensor malfunction is present. The output voltage of the FTP sensor should indicate atmospheric pressure with the fuel tank filler cap removed. The FTP sensor voltage will vary with atmospheric pressure. A typical voltage at sea level is between 3.5-3.8 volts.
-
This step determines if DTC P0451 is the result of a hard failure or an intermittent condition. Operating the vehicle in Check mode enhances the diagnostic capabilities of the PCM.
-
This step determines if there is trapped negative or positive pressure at the FTP sensor.
-
This step inspects for an open or a short in the FTP sensor 5-volt reference.
-
This step inspects for excessive resistance in the reference voltage circuit. If the test lamp does not illuminate at all, there is resistance in the circuit.
-
This step checks for an open or short in the FTP sensor input circuit.
-
This step checks for an open in the FTP sensor ground circuit.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Evaporative Emissions Hose Routing Diagram | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
Is the FTP sensor within the specified range? | 3.5-3.8 V | Go to Step 3 | Go to Step 4 | |
Does the DTC P0451 set? | -- | Go to Step 6 | Go to Diagnostic Aids | |
Is the FTP sensor within the specified range? | 3.5-3.8 V | Go to Step 5 | Go to Step 6 | |
5 | Repair the cause of the trapped vapor pressure in the fuel tank. Is the action complete? | -- | Go to Step 16 | -- |
Does the voltage equal the specified value? | 5 V | Go to Step 7 | Go to Step 8 | |
Does the test lamp illuminate at least dimly? | -- | Go to Step 9 | Go to Step 14 | |
8 | Repair the open in the reference voltage circuit of the FTP sensor between the sensor and SP108. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 16 | -- |
Measure the voltage of the FTP sensor input circuit on the harness side using a DMM. Is the voltage near the specified value? | 4.9 V | Go to Step 11 | Go to Step 10 | |
10 |
Was a repair necessary? | -- | Go to Step 16 | Go to Step 15 |
Probe the FTP sensor ground circuit on the harness side with a test lamp connected to B+. Does the test lamp illuminate? | -- | Go to Step 13 | Go to Step 12 | |
12 | Repair the open in the ground circuit of the FTP sensor between the sensor and SP108. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 16 | -- |
13 | Replace the FTP sensor. Refer to Evaporative Emission System Vacuum Leak Pump Replacement . Is the action complete? | -- | Go to Step 16 | -- |
14 | Repair the high resistance in the reference voltage circuit of the FTP sensor. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you complete the repair? | -- | Go to Step 16 | -- |
15 | Replace the powertrain control module (PCM). Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Is the action complete? | -- | Go to Step 16 | -- |
16 |
Does the DTC run and pass? | -- | Go to Step 17 | Go to Step 2 |
17 | 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 - Vehicle in Vehicle DTC Information | System OK |
