GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Express (CNG) 🢒 DTC 🢒 DTC P0121

Circuit Description

The throttle position (TP) sensor is a potentiometer. The gasoline Control Module (VCM / PCM or ECM) supplies about 5 volts on the TP sensor reference voltage circuit. The gasoline Control Module 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 gasoline Control Module and the Alternative Fuels ECM monitor the TP signal circuit voltage and determine the angle or opening of the throttle blade.

This DTC is used to indicate a throttle angle that 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.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

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

  1. This step indicates this sensor is currently functioning correctly. If you were sent to this diagnostic procedure from other service information, return to that procedure now.

    2.

  2. This step isolates the signal circuit from the other possible causes of the DTC condition.

    3.

  3. This step determines if the 5-volt reference circuit has the proper voltage. The amperage available on the 5-volt reference circuit should be about 180 mA.

    4.

  4. This step determines if the signal circuit is shorted to voltage or open.

    5.

  5. This step determines if the sensor ground circuit has good continuity. With the TP signal circuit jumpered to 5 volts and grounded through the sensor ground circuit terminal, very little voltage should be available at the TP signal circuit.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

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

--

Go to Step 2

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

2
  1. Turn the ignition ON, with the engine OFF.
  2. Using a scan tool, establish communication with the Alternative Fuels ECM.
  3. Observe the TP Sensor angle data parameter.

Does the scan tool display the throttle angle below the specified value?

2%

Go to Step 3

Go to Step 6

3

Depress the throttle slowly to wide open throttle (WOT).

Does the scan tool display the throttle angle ABOVE the specified value?

98%

Go to Step 4

Go to Step 6

4

Release the throttle.

Does the scan tool display the throttle angle BELOW the specified value?

2%

Go to Step 5

Go to Step 6

5

The fault is not present. Refer to Diagnostic Aids.

Are any DTCs stored that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

6

  1. Disconnect the TP sensor harness connector.

    2. Other DTCs may set.

  2. Connect a fused jumper between the TP signal circuit terminal and the 5-volt reference circuit terminal.
  3. Observe the TP Sensor voltage data parameter.

Does the scan tool display the TP sensor voltage greater than the specified value?

4.9 V

Go to Step 7

Go to Step 10

7

  1. Remove the fused jumper.
  2. Connect the J 39200 DMM test leads between the TP sensor 5-volt reference circuit terminal and a known good ground.
  3. Measure the voltage on the 5-volt reference circuit.

Is the voltage within the specified value range?

4.8 - 5.2 V

Go to Step 8

Go to Step 12

8

  1. Using a fused jumper connect the TP sensor signal circuit terminal to a known good ENGINE ground.

    2. If the jumper fuse opens, repair the signal circuit shorted to battery voltage.

    Other DTCs may set.

  2. Observe the TP Sensor voltage data parameter.

Does the scan tool display TP sensor voltage LESS than the specified value?

0.08 V

Go to Step 9

Go to Step 14

9

  1. Connect a fused jumper between the TP signal circuit terminal and the 5-volt reference circuit terminal.
  2. Connect another fused jumper between the TP sensor GROUND circuit terminal and the 5-volt reference circuit jumper.

    3. Other DTCs may set.

  3. Observe the TP Sensor voltage data parameter.

Does the scan tool display TP sensor voltage less than the specified value?

0.08 V

Go to Step 11

Go to Step 15

10
  1. Test the TP signal circuit for a short to ground or to the sensor ground circuit.
  2. Repair wiring as necessary.

Was a wiring repair made?

--

Go to Step 19

Go to Step 18

11
  1. Test the TP signal circuit for a short to voltage if the jumper fuse opens or a short to the 5-volt reference circuit.
  2. Inspect the TP sensor connector for poor terminal contact.
  3. Repair wiring or terminals as necessary.

Was a wiring or terminal repair made?

--

Go to Step 19

Go to Step 16

12
  1. Turn the ignition OFF.
  2. Disconnect both AF ECU connectors.
  3. Turn the ignition ON, with the engine OFF.
  4. Re-measure the voltage on the 5-volt reference circuit.

Is the voltage within the specified value range?

4.8-5.2 V

Go to Step 18

Go to Step 13

13
  1. Test the 5-volt reference circuit for a short to voltage - above 5.2 volts, a short to ground or high circuit resistance - below 4.8 volts.
  2. Repair wiring as necessary.

Was a wiring repair made?

--

Go to Step 19

Go to Step 17

14
  1. Inspect the TP Sensor Signal circuit for an open or high resistance condition between the Alternative Fuels ECM connector terminal and the circuit splice.
  2. Inspect the Alternative Fuels ECM connector and any in-line connectors, if applicable, for proper terminal contact.
  3. Repair wiring or terminals as necessary.

Was a repair necessary?

--

Go to Step 19

Go to Step 18

15
  1. Test the TP sensor ground circuit for high resistance or poor terminal contact at the gasoline Control Module (VCM/PCM or ECM) connector.
  2. Repair wiring or terminal contact as necessary.

Was a repair made?

--

Go to Step 19

Go to Step 17

16

Replace the TP Sensor. Refer to Throttle Position Sensor Replacement in the Engine Controls section of the G Van Service Manual.

Is the replacement complete?

--

Go to Step 19

--

17

Important: The following Control Module replacement procedures must all be completed before attempting to operate the vehicle.

The following procedures are located in the gasoline service manual.

  1. Replace the gasoline Control Module (VCM/PCM or ECM). Refer to Powertrain Control Module Replacement .
  2. Program the replacement Control Module with the correct vehicle software and calibration.

    3. Important: In order to perform the variation learn procedure, a switch-over to gasoline operation is necessary. Remove the CNG 20-amp fuse in order to perform the switch-over.

  3. Perform the Crankshaft Position System Variation Learn in Engine Controls.

Is the replacement complete?

--

Go to Step 19

--

18
  1. Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  2. Program the replacement Control Module with the correct vehicle software and calibration.

Is the replacement complete?

--

Go to Step 19

--

19
  1. Using a scan tool, clear the DTC information from BOTH Control modules.
  2. Operate the vehicle under the Conditions for Running the DTC.
  3. Using the scan tool parameter DTC STATUS, ensure that this DTC runs.

Did this DTC run and pass?

--

System OK

Go to Step 2