GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The throttle position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The TP sensor sends a voltage signal back to the engine control module (ECM) relative to the throttle plate opening. The voltage signal will vary from approximately 0.33 volts at closed throttle, to over 4.3 volts at wide open throttle (WOT).

The TP signal is used by the ECM for fuel control and for most of the ECM controlled outputs. The TP signal is one of the most important inputs used by the ECM for fuel control and most of the ECM controlled outputs. If the ECM detects a TP signal that is intermittently above the range of the TP sensor, DTC P1121 will be set.

Conditions for Setting the DTC

The TP sensor voltage indicates a throttle voltage intermittently greater than 4.9 volts.

Action Taken When the DTC Sets

    • The malfunction indicator lamp (MIL) will not illuminate.
    • The ECM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data.
    • A history DTC is stored.

Conditions for Clearing the MIL/DTC

    • A history will clear after 40 consecutive warm-up cycles in which the diagnostic runs without a fault.
    • DTCs can be cleared by using the scan tool.
    • Disconnect the ECM battery feed for more than 10 seconds.

Diagnostic Aids

Inspect for the following conditions:

    • A poor connection at the ECM--Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal-to-wire connection.
    • A damaged harness--Inspect the wiring harness for damage. If the harness appears OK, observe the throttle position display on the scan tool while moving connectors and wiring harnesses related to the TP sensor. A change in the display will indicate the location of the fault.

If DTC P1121 cannot be duplicated, reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Test Description

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

  1. The On-Board Diagnostic System Check prompts the technician to complete some basic checks and store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.

Step

Action

Yes

No

1

Perform an On-Board Diagnostic System Check.

Was the check performed?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Install a scan tool to the data link connector (DLC).
  2. Turn the ignition ON.

Is DTC P0123 also set?

Go to applicable DTC table

Go to Step 3

3

Inspect for poor sensor ground circuit terminal 1 connection at the throttle position (TP) sensor.

Is a repair necessary?

Go to Step 7

Go to Step 4

4

Inspect the TP signal circuit between TP sensor connector and the engine control module (ECM) for an intermittent short to voltage.

Is a repair necessary?

Go to Step 8

Go to Step 5

5

Inspect for a poor sensor ground terminal M64 at the ECM.

Is a repair necessary?

Go to Step 7

Go to Step 6

6

Inspect for an intermittent open or faulty splice in the sensor ground circuit.

Is a repair necessary?

Go to Step 8

Go to Diagnostic Aids

7

Repair or replace the faulty harness connector terminal for sensor ground circuit.

Did you complete the repair?

Go to Step 9

--

8

Repair the intermittent open/short circuit in the wiring harness as needed.

Did you complete the repair?

Go to Step 9

--

9

  1. Using the scan tool, clear the DTCs.
  2. Start the engine and idle at normal operating temperature.
  3. Operate the vehicle within the Conditions for Setting the DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic has run and passed?

Go to Step 10

Go to Step 2

10

Are any DTCs displayed that have not been diagnosed?

Go to applicable DTC table

System OK