GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 Bonneville 🢒 Body 🢒 Wiring Systems 🢒 DTC P0121

Circuit Description

The throttle position (TP) sensor is a potentiometer type sensor that varies resistance according to changes in the throttle plate position. The TP sensor wiring includes 3 circuits. The powertrain control module (PCM) supplies a regulated 5 volts to the sensor on a 5-volt reference circuit. The PCM supplies a ground on a low reference circuit. The TP sensor provides a signal voltage to the PCM, relative to the throttle plate opening, on the TP sensor signal circuit. With the throttle closed, the TP sensor voltage is low, near 0.5 volt with the throttle wide open, the TP sensor voltage is high, near 4.5 volts.

Within the PCM, the intake flow rationality diagnostic has the ability to determine an inaccurate input from the three systems that it monitors. Calibrated within the diagnostic are estimates for the manifold absolute pressure (MAP), the mass air flow (MAF), and the TP for all engine operating conditions. The diagnostic compares the estimated values to the actual sensor inputs. If the PCM detects that the TP sensor input is not within a calibrated limit of the estimated value, DTC P0121 sets.

Conditions for Running the DTC
    • DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0335, P0336, P0401, P0403, P0404, P0405 are not set.
    • The engine coolant temperature (ECT) is between 70-121°C (158-250°F).
    • The intake air temperature (IAT) is between -7 and +60°C (19-140°F).

Conditions for Setting the DTC

The PCM detects that the TP sensor voltage is not within the calibrated limit of the estimated value for more than 1 second.

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.

Test Description

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

  1. This step calculates the resistance in the 5-volt reference circuit.

    2.

  2. This step calculates the resistance in the low reference circuit.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

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

Observe the Capture Info with a scan tool.

Is DTC P0641 set?

--

Go to DTC P0641

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the TP sensor parameter with a scan tool.

Is the voltage less than the specified value with the throttle in the closed position?

0.85 V

Go to Step 4

Go to Step 6

4
  1. Observe the TP sensor parameter with the scan tool.
  2. Press the accelerator pedal slowly until the throttle is in the wide open position.
  3. Release the accelerator pedal slowly until the throttle is returned to the closed position.
  4. Repeat this procedure several times.

Does the TP sensor parameter increase steadily, when the pedal is depressed, to more than the first specified value and then decrease steadily, when the pedal is released, returning to less than the second specified value?

98%

1%

Go to Step 5

Go to Step 7

5

Important: The engine speed parameter must stabilize after each commanded change.

  1. Start the engine.
  2. Allow the engine to reach operating temperature.
  3. Allow the engine to idle.
  4. Turn OFF all accessories.
  5. Command the idle air control (IAC) valve to increase engine speed to 1,700 RPM and then decrease engine speed to 500 RPM with the scan tool RPM control function.

Is the engine speed within the specified value of the commanded RPM at each increment?

100 RPM

Go to Intermittent Conditions

Go to Idle Air Control (IAC) System Diagnosis

6

Inspect the throttle body for the following conditions:

    • Throttle blade binding or damaged
    • Cruise control cable binding or damaged
    • Throttle cable binding or damaged

Did you find and correct the condition?

--

Go to Step 15

Go to Step 7

7

Test for an intermittent and for a poor connection at the throttle position (TP) sensor. 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 8

8

  1. Disconnect the TP sensor harness connector.
  2. Measure the voltage from the 5-volt reference circuit of the TP sensor to a good ground, with a DMM. Note the measurement as "Supply Voltage".
  3. Connect test lamp and a DMM in series between the 5-volt reference and the low reference circuit of the TP sensor at the harness connector.
  4. Measure the amperage with the DMM. Note the measurement as "Amperage".
  5. Remove the DMM from the circuit.
  6. Connect the test lamp between the 5-volt reference circuit and the low reference circuit of the TP sensor, at the harness connector.

    7. Important: Before any calculations are performed, ensure that all measurements are converted to like units. For example, volts/lamps or millivolts/milliamps.

  7. Measure the voltage from the 5-volt reference circuit at the test lamp to a good ground, with the DMM. Note the measurement as "Load Voltage Drop.
  8. Subtract the "Load Voltage Drop" from the "Supply Voltage". Note the result as "Supply Voltage Drop".
  9. Divide the "Supply Voltage Drop" by the "Amperage".

Is the result more than the specified value?

5 ohms

Go to Step 10

Go to Step 9

9

Important: Before any calculations are performed, ensure that all measurements are converted to like units. For example, volts/lamps or millivolts/milliamps.

  1. Measure the voltage from the low reference circuit of the TP sensor at the test lamp to a good ground, wit the DMM. Note the result as "Low Reference Voltage Drop".
  2. Divide the "Low Reference Voltage Drop" by the "Amperage".

Is the result more than the specified value?

5 ohms

Go to Step 11

Go to Step 13

10

Test the 5-volt reference circuit between the powertrain control module (PCM) and the TP 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

11

Test the low reference circuit between the PCM and the TP 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

12

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.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

13

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

Did you complete the replacement?

--

Go to Step 15

--

14

Replace the PCM. Refer to Powertrain 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