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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD (LPG) 🢒 DTC 🢒 DTC P0123

Circuit Description

The throttle position (TP) sensor is a potentiometer. The powertrain control module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is pressed, the TP signal voltage rises to near the supplied 5 volts. As the throttle is released, the TP signal voltage drops to around 0.6 volts. The PCM and the Alternative Fuels ECM monitor the TP signal circuit voltage and determine the angle of the throttle blade.

This DTC sets when the TP sensor signal voltage is above the normal operating range of the sensor.

Conditions for Running the DTC

The engine is operating on alternative fuel.

Conditions for Setting the DTC

The Alternative Fuels ECM monitors a TP sensor signal voltage of greater than 4.9 volts for at least 1 second continuously.

Action Taken When the DTC Sets

    • A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
    • The Alternative Fuels ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
    • The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

    • The Last Test Failed status will clear when the DTC runs and passes.
    • The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
    • The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
    • All DTC Information can be cleared with a scan tool.

Test Description

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

  1. This step determines if the condition is present.

    2.

  2. This step determines if the TP signal is going high only when the throttle is opened.

    3.

  3. This step determines if the condition affects the PCM.

    4.

  4. This step determines if the TP signal circuit is the shorted condition.

    5.

  5. This step determines if an open sensor ground circuit is the cause of the condition.

    6.

  6. This step determines if the 5-volt reference circuit is shorted to a voltage. 5.2 volts is the maximum voltage that the PCM should supply on the 5-volt reference circuit.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2

  1. Observe the Freeze Frame and/or the Failure records data 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 also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data.
  5. Observe the Diagnostic Trouble Code (DTC) Information from the following control modules with a scan tool:
    6. • Alternative Fuels ECM
    • PCM

Does the DTC fail this ignition cycle?

--

Go to Step 4

Go to Step 3

3

  1. With a scan tool, establish communication with the PCM.
  2. Open and close the throttle while observing the TP Sensor data parameter.

Does the TP sensor data parameter ever indicate TP voltage greater than the specified value?

4.9 V

Go to Step 11

Go to Intermittent Conditions

4

  1. Turn the ignition ON, with the engine OFF.
  2. With a scan tool, establish communication with the PCM.
  3. Observe the TP Sensor data parameter.
  4. Fully depress the accelerator pedal.

Does the TP sensor data parameter indicate TP voltage greater than the specified value?

4.9 V

Go to Step 5

Go to Step 13

5

  1. Disconnect the TP Sensor connector. Refer to Throttle Position Sensor Replacement in Engine Controls in the C/K Pickup Service Manual.
  2. Observe the TP Sensor data parameter.

Does the TP sensor data parameter indicate TP voltage less than the specified value?

0.75 V

Go to Step 6

Go to Step 10

6

Probe the TP sensor ground circuit with a test lamp connected to B+.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 9

7

Measure the voltage on the 5-volt reference circuit of the TP sensor with a DMM.

Is the voltage less than the specified value?

5.2 V

Go to Step 11

Go to Step 8

8
  1. Inspect all 5-volt reference circuit for a short to voltage.

    2. Some 5-volt reference circuits are shared within the PCM. The short may be located on any of the 5-volt reference circuits that the PCM provides.

  2. Disconnect the Alternative Fuels ECM connector that contains the 5-volt reference circuit in order to determine if the ECM is the cause of the short to voltage.
  3. Repair wiring as necessary. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

9
  1. Test the sensor ground circuit for an open or high resistance condition between the sensor connector and the PCM connector terminal. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.
  2. Inspect for proper terminal contact at both the PCM connector and the TP sensor connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

10
  1. Test the TP signal circuit for a short to voltage. Refer to Circuit Testing in Wiring Systems in the C/K Pickup Service Manual.
  2. Disconnect the Alternative Fuels ECM connector that contains the TP signal circuit in order to determine if the ECM is the cause of the short to voltage.
  3. Repair wiring as necessary. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

11

Replace the TP Sensor. Refer to Throttle Position Sensor Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

--

Go to Step 14

--

12

Important: KL6 models require a switchover to gasoline with a scan tool in order to perform the Crankshaft Position Variation Learn Procedure.

KL8 models do not require the Crankshaft Position Variation Learn Procedure.

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

--

Go to Step 14

--

13

Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 14

--

14
  1. Use the scan tool in order to clear the DTCs from the following control modules:
    2. • Alternative Fuels ECM
    • PCM
  2. Turn OFF the ignition for 30 seconds.
  3. 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, select Capture Info in order to observe the stored information.

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

--

Go to Diagnostic Trouble Code (DTC) List

System OK