GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The throttle position (TP) sensor is used by the engine control module (ECM) to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with the following 3 circuits:

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

The ECM provides the TP sensor with 5 volts on the 5-volt reference circuit, and a ground on the low reference circuit. The TP sensor provides a signal voltage that changes relative to throttle plate angle. The TP sensor signal voltage at idle is less than 0.5 volt. The TP sensor voltage is typically near 0.0 volts at idle, but could be as high as 0.5 volts. The TP sensor voltage should increase to above 4 volts at wide open throttle (WOT). If the ECM detects an excessively high signal voltage, DTC P0123 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage

Conditions for Running the DTC

    • The ignition is ON or the engine is running.
    • DTC P0123 runs continuously once the above conditions are met.

Conditions for Setting the DTC

Important: The scan tool displays a voltage near 0.0 volts. Do not use the scan tool voltage parameter to determine if there is a TP sensor condition.

The ECM detects that the TP sensor signal voltage is more than 4.9 volts for a total of 2.5 seconds during a 5 second time period.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/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

  1. This step determines if there is a short to voltage or an open condition in the circuits.

  1. This step determines if the circuit has a short to voltage. A short to voltage can cause multiple DTCs to set.

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. Start the engine.
  2. Allow the engine to idle.
  3. Observe the TP sensor percentage parameter with a scan tool.

Is the percentage more than the specified value?

15%

Go to Step 4

Go to Step 3

3

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. 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.

Does the DTC fail this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4

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

Is the percentage less than the specified value?

15 %

Go to Step 5

Go to Step 7

5

Measure the voltage from the 5-volt reference circuit of the TP sensor to a good ground.

Does the voltage measure more than the specified value?

5.2 V

Go to Step 9

Go to Step 6

6

Measure the resistance between the low reference circuit of the TP sensor and the engine control module (ECM) housing with a DMM.

Is the resistance less than the specified value?

2 ohms

Go to Step 10

Go to Step 8

7

Test the signal circuit of 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 14

Go to Step 11

8

Important: The engine control module (ECM) and TP sensor may be damaged if the circuit is shorted to battery positive voltage.

Test the low reference circuit of the TP sensor for the following conditions:

    • A short to voltage
    • An open
    •  High resistance

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

9

Important: The 5-volt reference circuits are internally and externally connected at the controller. Other sensors that share the 5-volt reference circuit may also have DTCs set. Disconnecting a sensor on the shared 5-volt reference circuit may isolate a shorted sensor. Review the electrical schematic and diagnose the shared circuits and sensors.

Test the 5-volt reference circuit of 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 14

Go to Step 11

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 14

Go to Step 12

11

Test for an intermittent and for a poor connection at the engine control module (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 14

Go to Step 13

12

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

Did you complete the replacement?

--

Go to Step 14

--

13

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 14

--

14

  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 15

15

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