GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Epica 🢒 DTC 🢒 DTC P0121

Circuit Description

The throttle body assembly contains a throttle position (TP) sensor. The TP sensor is mounted within the throttle body assembly and is not serviceable. The TP sensor is a potentiometer which provides a signal voltage that changes relative to the throttle blade angle. The engine control module (ECM) supplies the TP sensor with a common 5-volt reference circuit, a common low reference circuit, and a signal circuit. The TP sensor signal voltage decreases from more than 4 volts at idle to less than 1 volt at wide open throttle (WOT).

The ECM uses the actual TP sensor signal to calculate the predicted mass airflow (MAF) value. The ECM then compares the actual MAF sensor signal to the predicted MAF value. If the ECM detects that the actual MAF sensor signal and the predicted MAF value differ by more than a predetermined amount, and the short term fuel trim remains steady, then the TP sensor signal is considered invalid and this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0121 Throttle Position (TP) Sensor Performance

Conditions for Running the DTC
    • DTCs P0122 and P0123 are not set.
    • The engine speed is between 1,000-3,500 RPM.
    • The engine load is stable.
    • The fuel system is operating in Closed Loop.
    • DTC P0121 runs continuously once the conditions above are met.

Conditions for Setting the DTC
    • The ECM detects that the TP sensor signal is more or less than threshold as compared to the MAF sensor signal.
    • The Short Term Fuel Trim remains steady.
    • The condition exists for more than 8 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.

Test Description

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

  1. This step verifies that a condition exists.

    2.

  2. This step determines if the TP sensor voltage is out of range.

    3.

  3. This step tests for high resistance in the 5-volt reference circuit of the TP sensor. If the DMM does not display more than the specified voltage, there is high resistance in the circuit.

    4.

  4. This step tests for high resistance in the low reference circuit of the TP sensor. Test for a condition in the low reference circuit if the resistance is more than the specified value with the ignition OFF.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine 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

  1. Allow the engine to reach operating temperature.
  2. Increase the engine speed to within the specified range for 1 minute.
  3. Observe the DTC Info with a scan tool.

Does the DTC fail this ignition?

1000-3500 RPM

Go to Step 5

Go to Step 3

3

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

Is the voltage within the specified range?

4.15-4.75 V

Go to Step 4

Go to Step 5

4
  1. Observe the Freeze Frame/Failure Records for this DTC.
  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 5

Go to Intermittent Conditions

5

  1. Turn OFF the ignition
  2. Disconnect the throttle body harness connector.
  3. Turn ON the ignition, with the engine OFF.

    4. Important: Use the J 35616-200 Test Lamp Kit for this test. If the J 35616-200 is not available, use a test lamp that measures more than 20 ohms.

  4. Connect a test lamp between the 5-volt reference circuit of the throttle position (TP) sensor and the engine control module (ECM) housing.
  5. Connect a DMM to the probe of the test lamp and the ECM housing. Refer to Measuring Voltage Drop in Wiring Systems.

Is the voltage more than the specified value?

4.8 V

Go to Step 6

Go to Step 9

6
  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 voltage parameter, with a scan tool.

Is the voltage more than the specified value?

4.8 V

Go to Step 7

Go to Step 8

7

  1. Turn OFF the ignition.

    2. Notice: Do NOT use a test lamp to test the continuity of the circuit. Damage to the control module may occur due to excessive current draw.

  2. Measure the resistance from the low reference circuit of the TP sensor to the ECM housing, with a DMM.

Is the resistance less than the specified value?

10 ohms

Go to Step 11

Go to Step 10

8

Test the signal circuit of TP sensor for a high resistance, or an open. 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

9

Test the 5-volt reference circuit of the TP sensor for a high resistance or an open. 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

10

Test the low reference circuit of the TP sensor for a high resistance or an open. 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 for shorted terminals and for poor connections at the throttle body assembly. 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 13

12

Test for shorted terminals and for poor connections at the throttle body assembly and at the ECM. 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 throttle body assembly. Refer to Throttle Body Assembly Replacement .

Did you complete the replacement?

--

Go to Step 15

--

14

Replace the ECM. Refer to Engine 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