GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Savana 🢒 DTC 🢒 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 a potentiometer type sensor with the following circuits:

    • A 5-volt reference circuit
    • A low reference circuit
    • A TP sensor signal circuit

The PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor 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 diagnostic trouble code (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. If the PCM detects the TP sensor voltage is out of the predicted range, DTC P0121 sets.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0122, P0123, P0506, P0507 are not set.
    • The engine is running for more than 2 minutes.
    • The engine coolant temperature (ECT) is more than 70°C (158°F).
    • The MAP is less than 43 kPa for a TP sensor skewed high test.
    • The MAP is more than 67 kPa for a TP sensor skewed low test.
    • The MAP is steady for 2 seconds or more.
    • The TP does not vary more than 1.5 percent for 2 seconds or more.

Conditions for Setting the DTC
    • The PCM detects that the TP sensor voltage is more than a predicted value when the MAP is less than 43 kPa.
        OR
    • The PCM detects that the TP sensor voltage is less than a predicted value when the MAP is more than 67 kPa.
    • The above conditions are met for 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

Inspect for the following conditions:

    • Vacuum hoses disconnected, damaged, or incorrectly routed
    • Manifold absolute pressure (MAP) sensor seal missing or damaged
    • Restrictions in the MAP sensor vacuum source
    • Intake manifold vacuum leaks

Did you find and correct the condition?

--

Go to Step 20

Go to Step 3

3

Important: The vehicle used for the comparison is not limited to the same type of vehicle as is being serviced. A vehicle known to provide an accurate reading is acceptable.

Do you have access to another vehicle in which the MAP sensor pressure can be observed with a scan tool?

--

Go to Step 4

Go to Step 5

4
  1. Turn ON the ignition, with the engine OFF.
  2. Observe the MAP sensor pressure with a scan tool.
  3. Observe the MAP sensor pressure in the known good vehicle with a scan tool.
  4. Compare the values.

Is the difference between the values less than the specified value?

3 kPa

Go to Step 6

Go to DTC P0106

5

Important: The Altitude vs. Barometric Pressure table indicates a pressure range for a given altitude under normal weather conditions. Weather conditions consisting of very low or very high pressure and/or very low or very high temperature may cause a reading to be slightly out of range.

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the MAP sensor pressure with a scan tool. Refer to Altitude Versus Barometric Pressure .
  3. The MAP sensor pressure should be within the range specified for your altitude.

Does the MAP sensor indicate the correct barometric pressure?

--

Go to Step 6

Go to DTC P0106

6
  1. Observe the MAP sensor pressure with a scan tool.
  2. Start the engine.

Does the MAP sensor pressure change?

--

Go to Step 7

Go to DTC P0106

7
  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 8

Go to Step 9

8
  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 Intermittent Conditions

Go to Step 10

9

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 20

Go to Step 10

10

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 20

Go to Step 11

11
  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 a 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".

Is the amperage equal to the specified value?

0 mA

Go to Step 15

Go to Step 12

12

  1. Remove the DMM from the circuit.
  2. Connect the test lamp between the 5-volt reference circuit and the low reference circuit of the TP sensor, at the harness connector.
  3. 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".

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

  4. Subtract the "load voltage drop" from the "supply voltage". Note the result as "supply voltage drop".
  5. Divide the "supply voltage drop" by the "amperage".

Is the result more than the specified value?

5 ohms

Go to Step 14

Go to Step 13

13

  1. Measure the voltage from the low reference circuit of the TP sensor at the test lamp to a good ground, with the DMM. Note the result as "low reference voltage drop".

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

  2. Divide the "low reference voltage drop" by the "amperage".

Is the result more than the specified value?

5 ohms

Go to Step 16

Go to Step 18

14

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 20

Go to Step 17

15

Test the low reference circuit between the PCM and the TP sensor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 20

Go to Step 17

16

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 20

Go to Step 17

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.

Did you find and correct the condition?

--

Go to Step 20

Go to Step 19

18

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

Did you complete the replacement?

--

Go to Step 20

--

19

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 20

--

20
  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 21

21

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