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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD (LPG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 DTC P0121

Circuit Description

The throttle position (TP) sensor is a potentiometer. The powertrain control module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is depressed, the TP signal voltage rises to near the supplied 5 volts. As the throttle is released, the TP signal voltage drops to around 0.6 volts. The PCM and the Alternative Fuels ECM monitor the TP signal circuit voltage and determine the angle of the throttle blade.

This DTC sets when the throttle angle is greater than calculated, based upon engine speed and engine manifold absolute pressure (MAP).

Conditions for Running the DTC
    • The engine is operating on alternative fuel.
    • The engine has been operating for greater than 2 seconds.
    • No Alternative Fuel ECM DTCs are set. DTCs P1215, P1432, and P1433 may be set.
    • The engine MAP is less than 45 kPa.
    • The TP sensor angle has changed less than 2 percent for 2 seconds continuously.

Conditions for Setting the DTC

The TP sensor angle is greater than the specified percentage at the given engine speed.

TP Sensor Angle

Engine Speed

30%

500 RPM

40%

1,500 RPM

50%

2,000 RPM

60%

3,000 RPM

70%

3,500 RPM

80%

4,000 RPM

90%

4,600 RPM

Action Taken When the DTC Sets

    • The Alternative Fuels ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
    • The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

    • The Last Test Failed status will clear when the DTC runs and passes.
    • The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
    • The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
    • All DTC Information can be cleared with a scan tool.

Test Description

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

  1. The step verifies that a PCM specific MAP sensor condition is not the cause of the DTC. The MAP sensor display should change when the engine is started. The MAP value should change with the engine speed. If a MAP sensor voltage changes, there is no condition with the MAP sensor, or PCM wiring.

    2.

  2. The step verifies that a PCM specific TP sensor condition is not the cause of the DTC. The TP sensor value should change with the engine speed. If the TP sensor voltage changes, there is no condition with the TP sensor.

    3.

  3. The step verifies that a Alternative Fuels ECM specific MAP sensor condition is not the cause of the DTC. The MAP sensor display should change when the engine is started. The MAP value should change with the engine speed. If a MAP sensor voltage changes, there is no condition with the MAP sensor, or Alternative Fuels ECM wiring.

    4.

  4. The step verifies that a specific TP sensor condition is not the cause of the DTC. The TP sensor value should change with the engine speed. If the TP sensor voltage changes, there is no condition with the TP sensor.

    5.

  5. This step determines if the Alternative Fuels ECM or wiring is the short to voltage.

    6.

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

    7.

  7. This step tests the TP sensor ground circuit. If the test lamp does not illuminate, test the ground circuit for an open or high resistance.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2

  1. Turn ON the ignition, with the engine OFF.
  2. With a scan tool, establish communications with the powertrain control module (PCM).
  3. Observe the MAP sensor voltage parameter.
  4. Start the engine.

Does the MAP sensor voltage parameter change value after you started the engine?

--

Go to Step 3

Go to DTC P0106

3

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

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

Below 1.0 V to above 4.0 V

Go to Step 4

Go to Step 7

4

  1. With a scan tool, establish communications with the Alternative Fuels ECM.
  2. Observe the MAP sensor voltage parameter.
  3. Start the engine.

Does the MAP sensor voltage parameter change value after you started the engine?

--

Go to Step 5

Go to DTC P0106

5

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

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

Below 1.0 V to above 4.0 V

Go to Step 6

Go to Step 14

6
  1. Observe the Freeze Frame and/or the Failure records data 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 and/or the Failure Records data.

Does the DTC fail this ignition cycle?

--

Go to Step 7

Go to Intermittent Conditions

7
  1. Disconnect the TP sensor. Refer to Throttle Position Sensor Replacement in Engine Controls in the C/K Pickup Service Manual.
  2. Observe the TP sensor voltage parameter with a scan tool.

Is the TP sensor voltage below the specified value?

0.75 V

Go to Step 9

Go to Step 8

8

  1. Turn OFF the ignition.
  2. Disconnect the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the TP sensor voltage parameter with a scan tool.

Is the TP sensor voltage below the specified value?

0.75 V

Go to Step 12

Go to Step 13

9

Connect a 3-amp fused jumper wire between the 5 volt reference circuit of the TP sensor and the signal circuit of the TP sensor.

Is the TP sensor voltage near the specified value?

5 V

Go to Step 10

Go to Step 15

10

Probe the TP sensor ground circuit with a test lamp connected to B+.

Does test lamp illuminate?

--

Go to Step 16

Go to Step 11

11

Test the TP sensor ground circuit for an open, or for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 17

12

Test the TP sensor signal circuit for a short to voltage between the Alternative Fuels ECM and the splice. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 21

13

Test the TP sensor signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 17

14

Test the TP sensor signal circuit for high resistance between the Alternative Fuels ECM and the splice. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 18

15

Test the TP sensor signal circuit and the TP sensor 5-volt reference circuit for an open, or for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 17

16

Test for an intermittent and for a poor connection at the TP sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 19

17

Test for an intermittent and for a poor connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 20

18

Test for an intermittent and for a poor connection at the Alternative Fuels ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 21

19

Replace the TP Sensor. Refer to Throttle Position Sensor Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

--

Go to Step 22

--

20

Important: KL6 models require a switchover to gasoline with a scan tool in order to perform the Crankshaft Position Variation Learn Procedure.

KL8 models do not require the Crankshaft Position Variation Learn Procedure.

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

--

Go to Step 22

--

21

Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 22

--

22
  1. Use the scan tool in order to clear the DTCs from the following control modules:
    2. • Alternative Fuels ECM
    • PCM
  2. Turn OFF the ignition for 30 seconds.
  3. 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 23

Go to Step 2

23

With a scan tool, select Capture Info in order to observe the stored information.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK