GM Service Manual Online
For 1990-2009 cars only
Table 1: P0451 Fuel Tank Pressure (FTP) Sensor Performance

Circuit Description

The control module monitors the fuel tank pressure (FTP) sensor signal in order to detect vacuum decay and excess vacuum during the evaporative emission (EVAP) diagnostic test. The control module supplies a 5-volt reference and a low reference circuit to the FTP sensor. This DTC sets if the engine control module (ECM) detects one of the following conditions:

    • The FTP sensor signal voltage is not within a calibrated range on a cold start-up.
    • The FTP sensor signal does not change by a calibrated amount when purge is active.
    • The FTP sensor signal oscillates more than a calibrated amount when vehicle speed is less than 30 km/h (50 mph).

The following table illustrates the relationship between FTP sensor signal voltage and the EVAP system pressure/vacuum.

FTP Sensor Signal Voltage

Fuel Tank Pressure

High, Approximately 1.5 Volts or More

Negative Pressure/Vacuum

Low, Approximately 1.5 Volts or Less

Positive Pressure

DTC Descriptor

This diagnostic procedure supports the following DTC.

DTC P0451 Fuel Tank Pressure (FTP) Sensor Performance

Conditions for Running the DTC

    • The fuel level is between 15-85 percent.
    • The start up engine coolant temperature (ECT) is less than 35°C (95°F).
    • The vehicle speed is less than 30 km/h (50 mph).
    • The ECM is commanding purge.
    • The ratio between ambient pressure and the calculated manifold pressure is less than 0.6.

Conditions for Setting the DTC

    • The ECM detects that the FTP sensor voltage on a cold start-up is less than 0.3 volts or more than 2.95 volts for 25 seconds.
    • The ECM does not detect a change in tank vacuum when purge duty cycle is more than 40 percent. The change in vacuum must be less than 0.063 kPa (0.25 inches H2O) when purge is active. This condition requires 3 failures to set the DTC for 10 seconds each time.
    • The ECM detects that the FTP sensor signal oscillates more than +/- 0.85 volts from a zero point for 25 seconds. For example, 1.5 volts = 0 pressure/vacuum change, the signal should not oscillate more than +/- 0.85 volts from this zero point. The zero point may vary between 1.0-2.5 volts. The vehicle speed must be less than 30 km/h (50 mph) for this test.

Action Taken When the DTC Sets

    • 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.

Conditions for Clearing the MIL/DTC

    • The malfunction indicator lamp (MIL) will turn OFF after 3 consecutive ignition cycles in which the diagnostic runs without a fault.
    • A history DTC will clear after 40 consecutive warm-up cycles without a fault.
    • Clear the DTCs with a scan tool.

Diagnostic Aids

    • Use the J 35616 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.
    • The lower connector of the ECM is connector C1 and the upper connector of the ECM is connector C2. Refer to Engine Controls Component Views .
    • A blocked or clogged EVAP canister vent may set this DTC.
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

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

  1. This step verifies that the condition is present.

  2. The ECM produces a measurable steady state amperage that provides the 5-volt reference to the FTP sensor. If the amperage on the 5-volt reference circuit is less than 80 mA, test the 5-volt reference circuit for an open or high resistance.

  3. This step tests the signal circuit of the FTP sensor.

P0451 Fuel Tank Pressure (FTP) Sensor Performance

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Turn on the ignition, with the engine OFF.
  2. Monitor the Fuel Tank Pressure (FTP) sensor parameter voltage with the scan tool.

Is the voltage within the specified range?

1.0-2.5 V

Go to Step 3

Go to Step 4

3

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn ON the ignition, with the engine OFF.
  2. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  3. Disconnect the fuel tank harness connector at the rear of the fuel tank.
  4. Set-up a DMM to test amperage on the 400 mA scale.
  5. Measure the amperage from the 5-volt reference circuit of the FTP sensor to a good ground.

Is the amperage more than the specified value?

80 mA

Go to Step 5

Go to Step 7

5

  1. Connect a 3-amp fused jumper wire between the 5-volt reference circuit of the FTP sensor and the signal circuit of the FTP sensor.
  2. Observe the FTP Sensor voltage parameter, with a scan tool.

Is the voltage more than the specified value?

4.8 V

Go to Step 6

Go to Step 8

6

  1. Turn OFF the ignition.
  2. Remove the ECM/TCM fuse from the underhood electrical center.
  3. Notice: Do NOT use a test lamp to test the continuity of the circuit. Damage to the control module may occur due to excessive current draw.

  4. Measure the resistance from the low reference circuit of the FTP sensor to a good ground.

Is the resistance less than the specified value?

5 ohms

Go to Step 10

Go to Step 9

7

Test the 5-volt reference circuit of the FTP sensor for high resistance between the harness connector and the control module. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

8

Test the signal circuit of the FTP sensor for high resistance between the harness connector and the control module. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

9

Test the low reference circuit of the FTP sensor for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

10

  1. Remove the fuel tank. Refer to Fuel Tank Replacement .
  2. Inspect the fuel tank wiring harness for the following conditions:
  3. • Damaged wiring
    • High resistance in the FTP harness from the fuel tank wiring harness connector to the FTP sensor
  4. Repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 11

11

Test for shorted terminals and for a poor connection at the FTP sensor harness connector, and the fuel tank harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

12

Test for shorted terminals and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

13

Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement .

Did you complete the replacement?

--

Go to Step 15

--

14

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

15

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 16

16

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK