GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC C Pickup - 2WD (CNG) 🢒 DTC 🢒 DTC P0121 Throttle Position (TP) Sensor Performance

Object Number: 357774 Size: MF
Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

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 AF ECU 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 more than calculated, based upon engine speed and engine MAP.

Conditions for Running the DTC
    • The engine is operating on alternative fuel.
    • The engine has been operating for more than 2 seconds.
    • No AF ECU DTCs are set. DTCs P1215, P1432 and P1433 may be set.
    • The engine MAP is less than 55 kPa.
    • The TP sensor angle has changed less than 2 percent for 2 seconds continuously.

Conditions for Setting the DTC

The AF ECU monitors a throttle angle that is more than calculated based upon the engine speed and the engine MAP.

Action Taken When the DTC Sets

    • A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
    • The AF ECU will illuminate the MIL and store the DTC as History after TWO consecutive drive trips that the diagnostic runs and fails.
    • The AF ECU 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 AF ECU 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 using a scan tool.
    • Interruption of Control Module (AF ECU / VCM / PCM or ECM) power or ground MAY clear DTC Information. Clearing DTC Information with this method is inconsistent and should not be performed.

Diagnostic Aids

Fault Not Present

Fault Not Present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions for additional information. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in the OBD System Check.

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 current 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) grounded through the sensor ground circuit terminal, very little voltage should be available at the TP signal circuit.

Step

Action

Value(s)

Yes

No

1

Did you perform the AF Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check

2
  1. Turn ON the ignition, with the engine OFF.
  2. Using a scan tool establish communication with the AF ECU.
  3. Observe the TP sensor angle data parameter.

Is the throttle angle less than the specified value?

2 %

Go to Step 3

Go to Step 6

3

Depress the throttle slowly to Wide Open Throttle (WOT).

Is the throttle angle more than the specified value?

98 %

Go to Step 4

Go to Step 6

4

Release the throttle.

Is the throttle angle less than 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 1 amp fused jumper between the TP signal circuit terminal and the 5 volt reference circuit terminal.
  3. Observe the TP sensor voltage data parameter.

Is the TP sensor voltage more than the specified value?

4.9 V

Go to Step 7

Go to Step 10

7

  1. Remove the fused jumper.
  2. Connect a digital multimeter (DMM) 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 range?

4.8-5.2 V

Go to Step 8

Go to Step 12

8

  1. Using a 1 amp 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 short to battery voltage.

    Other DTCs may set.

  2. Observe the TP sensor voltage data parameter.

Is the TP sensor voltage less than the specified value?

0.08 V

Go to Step 9

Go to Step 14

9

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

    3. Other DTC's may set.

  3. Observe the TP sensor voltage data parameter.

Is the 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 the 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 (jumper fuse opens) or a short to the 5 volt reference circuit.
  2. Repair the wiring as necessary.

Was a wiring repair made?

--

Go to Step 19

Go to Step 16

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

Is the voltage within the specified 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 (more than 5.2 volts), a short to ground, or high circuit resistance (less than 4.8 volts).
  2. Repair the 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 AF ECU connector terminal and the circuit splice.
  2. Inspect the AF ECU connector and any in-line connectors (if applicable) for proper terminal contact.
  3. Repair the 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 faulty terminal contact at the gasoline control module (VCM/PCM or ECM) connector.
  2. Repair the 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 l.

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.

  1. Replace the gasoline control module (VCM/PCM or ECM). Refer to VCM Replacement/Programming .
  2. Program (flash) 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 .

    4. All of the above procedures are located in the gasoline service manual.

Is the replacement complete?

--

Go to Step 19

--

18
  1. Replace the AF ECU. Refer to Engine Control Unit Replacement .
  2. Program (flash) 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