GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 DTC P0121

Circuit Description

The throttle position (TP) sensor is used by the powertrain control module (PCM) to determine the throttle plate angle for various engine management systems. The TP sensor is potentiometer sensor with a 5-volt reference circuit, a low reference circuit , and a signal circuit. The PCM provides the TP sensor with a 5-volt reference circuit and a low reference circuit. Rotation of the TP sensor from the closed throttle position to the wide open throttle (WOT) position provides the PCM with a signal voltage from less than 1 volt to more than 4 volts through the TP sensor signal circuit. When the conditions for running this DTC are met, the PCM will use the manifold absolute pressure (MAP) sensor in order to determine if the predicted operating range of the TP sensor is correct. When the TP sensor voltage is out of this predicted range, DTC P0121 sets.

Conditions for Running the DTC
    • DTCs P0122, P0123, P0107, P0108, P1123, and P1122 are not set.
    • The engine run time is more than 2 minutes.
    • The engine coolant temperature is more than 70°C (158°F).
    • The MAP is less than 40 kPa.
        OR
    • The MAP is more than 67 kPa.
    • The MAP is steady.

Conditions for Setting the DTC
    • The predicted throttle angle does not match the actual throttle angle.
    • All conditions are present for 2 seconds.

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 control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

A skewed MAP sensor may cause this DTC to set. If the TP sensor is operating properly, and if this DTC continues to set, test the MAP sensor for proper operation.

The PCM 5-volt reference circuits are internally connected within the PCM. If all the TP sensor circuits are OK, inspect the MAP sensor for malfunctions.

If it is determined that the DTC occurs intermittently, performing the DTC P1121 diagnostic test may isolate the cause of the fault. Refer to DTC P1121 .

Test Description

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

  1. If DTC P1635 sets at the same time, the 5-volt reference circuit is either shorted to a ground, or is shorted to a voltage. The 5-volt reference circuit is internally connected within the PCM.

    2.

  2. The MAP value should change with the engine speed. If a MAP sensor voltage does not change, this test indicates a map sensor fault.

    3.

  3. This step tests for an intermittent connection at the sensor.

    4.

  4. Using Freeze Frame/Failure Records data may help to locate an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame/Failure Records data can help to determine how many miles have occurred since the DTC set. The Fail Counter and the Pass Counter can also help to determine how many ignition cycles have occurred since the diagnostic reported a pass and/or a fail. In order to isolate when the DTC failed, operate the vehicle within the same Freeze Frame conditions that you observed. These conditions include the RPM, the vehicle load, the vehicle speed, and the temperature.

    5.

  5. This tests the 5-volt reference circuit, the signal circuit, and the PCM. If the scan tool displayed 5 volts, the TP sensor circuits are OK.

Step

Action

Values

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. Install the scan tool.
  2. Idle the engine.
  3. Monitor the DTC Information option using the scan tool.

Did DTC P1404 or P1635, P1639, P0107, P0108 also set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Start the engine.
  2. Monitor the MAP sensor voltage.
  3. Increase the engine speed to the specified value.

Does the MAP sensor voltage change when the engine speed changed?

2000 RPM

Go to Step 8

Go to Step 4

4

  1. Turn ON the ignition, with the engine OFF.
  2. Manually move the MAP sensor electrical connector, the harness and the PCM connectors while you observe the scan tool display.

Is the MAP value affected by moving the harness or the connectors?

--

Go to Step 16

Go to Step 5

5

  1. Remove the MAP sensor from the intake manifold, and leave the electrical harness connected. Refer to Manifold Absolute Pressure Sensor Replacement .
  2. Connect a hand-operated vacuum pump to the MAP sensor.
  3. Observe and/or record the MAP display while slowly applying vacuum up to 20 in Hg, as indicated on the pump gauge. Each 1 inch of vacuum applied should result in a 3 to 4 kPa drop in the MAP sensor value on the scan tool. The value should change smoothly with each increase in vacuum.

Did the MAP value change smoothly through the entire range of the test, without any erratic readings?

--

Go to Step 6

Go to Step 17

6

Apply 20 inches Hg vacuum to the MAP sensor.

Is the MAP sensor reading the same or less than the specified value?

34  kPa

Go to Step 7

Go to Step 17

7

Disconnect the vacuum source from the MAP sensor.

Does the MAP sensor reading return to the original value that you observed in step 5?

--

Go to Step 8

Go to Step 17

8

  1. Turn ON the ignition, with the engine OFF.
  2. Monitor the TP sensor voltage while moving the accelerator pedal from a closed throttle to a wide open throttle.

Does the TP sensor voltage go from below the first value to above the second value?

1-4 V

Go to Step 9

Go to Step 10

9

  1. Turn ON the ignition, with the engine OFF.
  2. Review the Freeze Frame/Failure Records data for this DTC, and record the parameters.
  3. Turn OFF the ignition for 15 seconds.
  4. Re-install the MAP sensor, and re-install any other disconnected components.
  5. Start the engine.
  6. Operate the vehicle within the conditions required for this diagnostic to run. Attempt to duplicate the conditions recorded in Freeze Frame/Failure Records. Special operating conditions that you must meet before the PCM will run this diagnostic are listed in Conditions for Running the DTC.
  7. Select the Diagnostic Trouble Code (DTC) option, the Specific DTC option, then enter the DTC number which was set using the scan tool.

Does the scan tool indicate that this diagnostic failed this ignition?

--

Go to Step 10

Go to Diagnostic Aids

10
  1. Disconnect the TP sensor.
  2. Observe the TP sensor display on the scan tool.

Is the TP sensor voltage near the specified value?

0 V

Go to Step 11

Go to Step 13

11

Jumper the TP sensor 5-volt reference circuit to the TP sensor signal circuit using a fused jumper wire. Refer to Using Fused Jumper Wires in Wiring Systems.

Is the TP sensor voltage near the specified value?

5 V

Go to Step 12

Go to Step 14

12

Replace the TP sensor. Refer to Throttle Position Sensor Replacement .

Did you complete the repair?

--

Go to Step 18

--

13
  1. Test the following components for the following conditions:
    2. • Test the TP sensor signal circuit for a short to voltage.
    • Test the TP sensor low reference circuit for high resistance between the PCM and the TP sensor.
    • Test the TP sensor low reference circuit for a poor connection.
  2. If you find a condition, repair the condition as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 15

14
  1. Test for the following conditions:
    2. • Test the TP sensor signal circuit or the 5-volt reference circuit for a poor connection. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • Test TP signal circuit or the 5-volt reference circuit for high resistance between the PCM and the TP sensor.
  2. If you find a condition, repair the condition as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 15

15

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the repair?

--

Go to Step 18

--

16

Locate and repair the affected circuits.

Did you complete the repair?

--

Go to Step 18

--

17

Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement .

Did you complete the replacement?

--

Go to Step 18

--

18
  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 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