GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2003 🢒 DeVille 🢒 Engine 🢒 Engine Controls - 4.6L 🢒 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 three circuits:

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

The PCM provides the TP sensor with 5 volts on the 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 greater than 4 volts through the TPsensor signal circuit. When the conditions for running this DTC are met, the PCM will use the manifold absolute pressure (MAP) sensor 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
    • DTC P0106, P0107, P0108, P0122, P0123, P0506, P0507 are not set.
    • The engine is running for more than 30 seconds.
    • The engine coolant temperature (ECT) is more than 0°C (32°F).
    • The MAP reading is less than 55 kPa for a TP sensor skewed high test.
    • The MAP reading is more than 64 kPa for a TP sensor skewed low test.
    • The MAP is steady for 5 seconds or more.

Conditions for Setting the DTC
    • The PCM detects that the TP sensor voltage is more than the predicted value when the MAP sensor pressure is less than 55 kPa.
        OR
    • The PCM detects that the TP sensor voltage is less than the predicted value when the MAP sensor pressure is more than 64 kPa.
    • The above conditions are met for 5 seconds.

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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

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

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

Did you find and correct the condition?

--

Go to Step 25

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 lowor 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 throttle position (TP) sensor parameter with the scan tool.

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

0.90 V

Go to Step 8

Go to Step 10

8
  1. Observe the TP sensor parameter with the scan tool.
  2. Depress 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 9

Go to Step 11

9

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

  1. Start the engine
  2. Allow the engine to idle
  3. Turn OFF all accessories
  4. Command the idle air control (IAC) valve to decrease engine speed to 500 RPM with the scan tool RPM control function. Refer to Scan Tool Output Controls

Is the engine speed within the specified value of the commanded RPM?

100 RPM

Go to Intermittent Conditions

Go to Idle Air Control (IAC) System Diagnosis

10

Inspect the throttle body for the following conditions:

    • A binding or damaged throttle blade
    • A binding or damaged throttle cable

Did you find and correct the condition?

--

Go to Step 25

Go to Step 11

11
  1. Disconnect the TP sensor electrical connector.
  2. Observe the TP sensor parameter with the scan tool.

Is the voltage less than the specified value?

0.16 V

Go to Step 12

Go to Step 16

12

Measure the voltage from the 5-volt reference circuit of the TP sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage more than the specified value?

5.2 V

Go to Step 17

Go to Step 13

13

Probe the 5-volt reference circuit of the TP sensor with a test lamp that is connected to a good ground. Refer to Circuit Testing and Probing Electrical Connectors in WiringSystems.

Is the test lamp OFF?

--

Go to Step  18

Go to Step 14

14
  1. 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.
  2. Observe the TP sensor parameter with the scan tool.

Is the voltage more than the specified value?

4.9 V

Go to Step 15

Go to Step 19

15
  1. Turn OFF the ignition.
  2. Remove the jumper wire.
  3. Connect a jumper wire between each of the terminals in the TP sensor harness connector and the corresponding terminal at the TP sensor. Refer to Using Connector Test Adapters in Wiring Systems.
  4. Turn ON the ignition, with the engine OFF.
  5. Measure the voltage from the low reference circuit of the TP sensor at the jumper wire terminal to a good ground with a DMM. Refer to Measuring Voltage Drop in Wiring Systems.

Is the voltage more than the specified value?

0.2 V

Go to Step 20

Go to Step 21

16

Important: Disconnecting the powertrain control module (PCM) may eliminate the short during testing.

Test the TP sensor signal circuit between the PCM and the TP sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 24

17

Important: Disconnecting the PCM may eliminate the short during testing.

Test all of the branches of the 5-volt reference circuit that are shared with the TP sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 24

18

Test the 5-volt reference circuit between the PCM and the TP sensor for an open or for a high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

19

Test the TP sensor signal circuit between the PCM and the TP sensor for the following conditions:

    • A short to ground
    • An open
    • A high resistance

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

20

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

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

21

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.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 23

22

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 25

Go to Step 24

23

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

Did you complete the replacement?

--

Go to Step 25

--

24

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 25

--

25
  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 26

26

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