GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The DTC P0123 Throttle Position (TP) Sensor 1 Circuit High Voltage diagnostic detects an out of range sensor signal. The TP sensor 1 output provides the powertrain control module (PCM) with a signal voltage that is proportional to the throttle plate movement. The TP sensor used with the throttle actuator control (TAC) system is actually a Hall Effect switch. The Hall Effect switch is surrounded by a magnetic yoke that induces a flux in the magnetic field when the throttle shaft is rotated. The PCM provides a 5-volt reference voltage to the sensor circuits in the TAC module. An integrated circuit (IC) receives and converts the magnetic pulses into 2 separate TP signal, each with their own characteristics. Both TP sensor signal voltages are low at closed throttle and increase as the throttle opens. TP sensor 1 determines the actual throttle valve position. TP sensor 2 has a voltage that is more than twice that of TP sensor 1 at idle. TP sensor 2 provides a backup value for TP sensor 1.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0123 Throttle Position (TP) Sensor 1 Circuit High Voltage

Conditions for Running the DTC

    • The ignition is ON or the engine is running.
    • DTC P0123 runs continuously once the above condition has been met.

Conditions for Setting the DTC

The TP sensor 1 input to the PCM is more than 4.5 volts for at least 2 seconds.

Action Taken When the DTC Sets

    • The PCM illuminates the malfunction indicator lamp (MIL).
    • The PCM stores the conditions that were present when the DTC set as Freeze Frame data.
    • The engine will operate in the reduced power mode and the TP sensor angle will default to 19 percent.

Conditions for Clearing the MIL/DTC

    • The PCM turns OFF the MIL on the third consecutive trip in which the diagnostic runs without a fault.
    • The PCM clears a History DTC after 40 consecutive warm-up cycles without a fault.
    • You use the scan tool Clear DTC Information function.

Diagnostic Aids

    • On the scan tool, a good TP sensor 1 will typically display a TP angle of 8-10 percent at idle, and 76-77 percent at wide open throttle (WOT).
    • An intermittent malfunction may be caused by fault in the TAC module electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .
    • Repair any electrical circuit faults that were found. Refer to Wiring Repairs .
    • The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

Test Description

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

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

  2. This step determines if DTC P0123 is the result of a hard failure or an intermittent condition.

  3. The TP sensor signal circuit carries a 5-volt bias that can be measured with the DMM. If the measured voltage is near zero there is an open or a faulty PCM. If the voltage is near 5 volts, the circuit is OK. If the voltage is more than 5 volts, there is a short to voltage on the input circuit.

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 - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

With the throttle closed, select the TP sensor parameter on the scan tool.

Is the TP sensor percentage within the specified value?

7-11%

Go to Step 3

Go to Step 4

3

  1. Perform the scan tool Clear DTC Information function.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Freeze Frame conditions as specified.

Does DTC P0123 set?

--

Go to Step 4

Go to Diagnostic Aids

4

Is a DTC P0453 also set?

--

Go to Step 8

Go to Step 5

5

  1. Disconnect the throttle actuator control (TAC) module electrical connector.
  2. Connect a test lamp to B+ and the ground circuit of the throttle position (TP) sensor.

Did the test lamp illuminate?

--

Go to Step 6

Go to Step 9

6

  1. Connect a DMM to the TP sensor 1 input circuit at the harness connector.
  2. Turn ON the ignition, with the engine OFF.

Is the input circuit voltage within the specified value?

4.9-5.1 V

Go to Step 10

Go to Step 7

7

  1. Test for an open or a short to voltage in the TP sensor 1 input circuit.
  2. Repair the circuit as necessary. Refer to Wiring Repairs .

Did you find and correct a condition?

--

Go to Step 14

Go to Step 12

8

  1. Test for an open in the TP sensor 1 ground circuit between SP108 and the powertrain control module (PCM).
  2. Repair the circuit as necessary. Refer to Wiring Repairs .

Did you find and correct a condition?

--

Go to Step 14

Go to Step 12

9

  1. Test for an open in the TP sensor 1 ground circuit between SP108 and the TAC module.
  2. Repair the circuit as necessary. Refer to Wiring Repairs .

Did you find and correct a condition?

--

Go to Step 14

Go to Step 10

10

  1. Test for an intermittent or a poor electrical connection at the TAC module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .
  2. Repair as necessary.

Did you find and correct a condition?

--

Go to Step 14

Go to Step 11

11

The TP sensor in the TAC module is faulty. Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement .

Did you complete the replacement?

--

Go to Step 14

--

12

  1. Test for an intermittent or a poor electrical connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .
  2. Repair as necessary.

Did you find and repair a condition?

--

Go to Step 14

Go to Step 13

13

Replace the PCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 14

--

14

  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 15

Go to Step 2

15

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK