GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The Throttle Position (TP) sensor provides a voltage signal which changes relative to the throttle blade angle. The signal voltage varies from about 0.5 volt at idle to about 4.0 volts at Wide Open Throttle (WOT).

The VCM uses the TP signal, one of the most important inputs, for fuel control and for most of the VCM control outputs.

Each time the voltage drops below 1.25 volts and stops, the VCM assumes this value as the 0 throttle angle, and the VCM measures the percent throttle from this point on. The DTC P0123 is a type A DTC. The DTC P1122 is a type D DTC.

Conditions for Setting the DTC

The DTC will set under the following conditions:

    • The engine is running
    • The Throttle Position (TP) sensor signal is greater than 4.9 volts

Action Taken When the DTC Sets

Important: The scan tool will not indicate default values.

When the P0123 is reported and 1 failure has occurred, the VCM sets the DTC P0123, and the VCM turns ON the MIL.

Conditions for Clearing the MIL/DTC

The VCM turns OFF the MIL after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle) or the scan tool clearing feature has been used.

Diagnostic Aids

The VCM pins GRY 12 and GRY 27 are spliced together inside of the VCM. When checking one of these circuits for a problem, also check the other circuit and the components on the circuit.

The scan tool reads the throttle position in volts. The scan tool should read about 0.45 to 0.85 volt with the throttle closed and the ignition switch turned ON or at idle. The voltage should increase at a steady rate as the throttle is moved toward the Wide Open Throttle (WOT).

Also, some scan tools will read the throttle angle. 0% = closed throttle. 100% = WOT.

Scan the TP sensor while depressing the accelerator pedal with the engine turned OFF and the ignition turned ON. The display should vary from below the 1.25 volts (1250 mV) when the throttle is closed to over 4.5 volts (4500 mV) when the throttle is held at WOT position.

A DTC P0123 will result if the TP ground circuit is open or the TP signal circuit is shorted to the voltage.

A DTC P0122 will result if there is an open or a short to ground in the 5 volt reference circuit or the TP sensor signal circuit.

Refer to Intermittent Conditions .

Test Description

Number(s) below refer to step number(s) on the Diagnostic Table.

  1. If the TP signal is greater than 4.8 volts, then the fault is present.

  2. With the TP sensor disconnected, the TP signal voltage should go low if the VCM and wiring are OK.

  3. The VCM pins GR12 and GR27 are spliced together inside the VCM. This step isolates the two pins from one anther.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Important: Before clearing the DTCs use the scan tool to record the Freeze Frame and the Failure Records data. This data will be lost when the Clear Info function is used.

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Connect the scan tool.
  2. Turn the ignition ON, engine OFF.

Does the scan tool display a TP sensor voltage greater than the specified value?

4.8 V

Go to Step 3

Go to Step 4

3

  1. Turn the ignition OFF.
  2. Disconnect the TP sensor electrical connector.
  3. Turn the ignition ON.

Does the scan tool display a TP sensor voltage less than the specified value?

0.2 V

Go to Step 5

Go to Step 8

4

The DTC is intermittent. If no addition DTCs are stored, refer to Diagnostic Aids.

Are any addition DTCs stored?

--

Go to Applicable DTC Table

--

5

With a DVM connected to ground, probe the 5 volt reference circuit (at the TP sensor harness connector).

Is the voltage greater than the specified value?

5.2 V

Go to Step 9

Go to Step 6

6

Probe the TP sensor ground circuit (at the TP sensor connector) with a test light to B+.

Is the test light ON?

--

Go to Step 7

Go to Step 13

7

Replace the TP sensor.

Is the action complete?

--

Go to Step 18

--

8

Check for a short to voltage on the TP sensor signal circuit.

Was a problem found?

--

Go to Step 14

Go to Step 17

9

  1. Turn the ignition OFF.
  2. Disconnect the VCM gray connector.
  3. Turn the ignition ON
  4. With a DVM connected to ground, measure the voltage on the 5 volt reference circuit (at the VCM harness connector).

Is the voltage greater than the specified value?

5.2 V

Go to Step 12

Go to Step 10

10

With a DVM connected to ground, measure the voltage on the VCM connector terminal GR27.

Is the voltage greater than the specified value?

5.2 V

Go to Step 11

Go to Step 17

11

  1. Disconnect the EGR valve electrical connector.
  2. Measure the voltage on the 5 volt reference circuit (at the VCM harness connector GR27).

Is the voltage greater than the specified value?

5.2 V

Go to Step 16

Go to Step 15

12

Repair the short to voltage on the 5 volt reference circuit.

Is the action complete?

--

Go to Step 18

--

13

Check the sensor ground circuit for an open.

Was a problem found?

--

Go to Step 14

Go to Step 17

14

Repair as necessary.

Is the action complete?

--

Go to Step 18

--

15

Replace the EGR valve.

Is the action complete?

--

Go to Step 18

--

16

Repair short to voltage on the 5 volt reference circuit (terminal GR27).

Is the action complete?

--

Go to Step 18

--

17

Replace the VCM.

Important: If the VCM is faulty, the new VCM must be programmed. Refer to VCM Replacement/Programming .

Is the action complete?

--

Go to Step 18

--

18

  1. Using the scan tool, select DTC, Clear Info.
  2. Start the engine and idle until normal operating temperature is reached.
  3. Select DTC, Specific then enter the DTC number which was set.
  4. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic Ran and Passed?

--

Go to Step 19

Go to Step 2

19

Using the scan tool, select Capture Info, Review Info.

Are any addition DTCs displayed that have not been diagnosed?

--

Go to Applicable DTC Table

System OK