GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Savana (CNG) 🢒 DTC 🢒 DTC P0123

Circuit Description

The throttle position (TP) sensor is a potentiometer. The gasoline Control Module (VCM / PCM or ECM) supplies about 5 volts on the TP sensor reference voltage circuit. The gasoline Control Module also supplies a ground circuit to the TP sensor. When the throttle is depressed, 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 gasoline Control Module and the Alternative Fuels ECM monitor the TP signal circuit voltage and determine the angle or opening of the throttle blade.

This DTC is used to indicate a TP sensor signal voltage that 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.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

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

  1. This step determines if the failure condition affects the gasoline Control module.

    2.

  2. This step determines if the failure condition is present.

    3.

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

    4.

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

    5.

  5. This step determines if an open sensor ground condition is present.

    6.

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

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

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

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Go to Step 2

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

2

  1. Turn the ignition ON, with the engine OFF.
  2. Using the scan tool, establish communication with the gasoline Control Module (VCM/PCM or ECM).
  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 6

Go to Step 3

3

  1. Using the scan tool, establish communication with the Alternative Fuels ECM.
  2. Observe the TP Sensor data parameter.

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

4.9 V

Go to Step 12

Go to Step 4

4

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 6

Go to Step 5

5

The fault is not present. Refer to Diagnostic Aids.

Are any DTCs stored that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

6

  1. Disconnect the TP Sensor connector.
  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 7

Go to Step 11

7

Connect the J 35616-200 Unpowered Test Lamp between the sensor ground circuit terminal and BATTERY POSITIVE.

Is the test lamp illuminated?

--

Go to Step 8

Go to Step 10

8

  1. Connect the J 39200 DMM between the 5-volt reference circuit terminal and GROUND.
  2. Measure the voltage on the 5-volt reference circuit.

Is the voltage LESS than the voltage specified?

5.2 V

Go to Step 15

Go to Step 9

9
  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.

Was a wiring repair made?

--

Go to Step 18

Go to Step 16

10
  1. Inspect the sensor ground circuit for an open or high resistance condition between the sensor connector and the gasoline Control Module (VCM/PCM or ECM) connector terminal.
  2. Inspect for proper terminal contact at both the gasoline Control Module connector and the TP sensor connector.
  3. Repair wiring or terminal contact as necessary.

Was a wiring or terminal repair necessary?

--

Go to Step 18

Go to Step 16

11
  1. Test the TP signal circuit for a short to voltage.
  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.

Was a wiring repair made?

--

Go to Step 18

Go to Step 16

12
  1. Turn the ignition OFF.
  2. Disconnect the Alternative Fuels ECM connector that contains the TP signal circuit.
  3. Disconnect the TP sensor connector.
  4. Test the TP signal circuit for continuity with a DMM.

Does the DMM indicate continuity?

--

Go to Step 14

Go to Step 13

13

Repair the open TP signal circuit between the TP signal circuit splice and the Alternative Fuels ECM.

Is the circuit repair complete?

--

Go to Step 18

--

14
  1. Inspect for proper terminal contact at the Alternative Fuels ECM connector.
  2. Repair terminal contact as necessary.

Was a terminal repair made?

--

Go to Step 18

Go to Step 17

15

Replace the TP Sensor. Refer to Throttle Position Sensor Replacement in the Engine Controls section of the G Van Service Manual.

Is the replacement complete?

--

Go to Step 18

--

16

Important: The following Control Module replacement procedures must all be completed before attempting to operate the vehicle.

The following procedures are located in the gasoline service manual.

  1. Replace the gasoline Control Module (VCM/PCM or ECM). Refer to Powertrain Control Module Replacement .
  2. Program the replacement Control Module with the correct vehicle software and calibration.

    3. Important: In order to perform the variation learn procedure, a switch-over to gasoline operation is necessary. Remove the CNG 20-amp fuse in order to perform the switch-over.

  3. Perform the Crankshaft Position System Variation Learn in Engine Controls.

Is the replacement complete?

--

Go to Step 18

--

17
  1. Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  2. Program the replacement Control Module with the correct vehicle software and calibration.

Is the replacement complete?

--

Go to Step 18

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18
  1. Using a scan tool, clear the DTC information from BOTH Control Modules.
  2. Operate the vehicle under the Conditions for Running the DTC.
  3. Using the scan tool parameter DTC STATUS, ensure that this DTC runs.

Did this DTC run and pass?

--

System OK

Go to Step 2