GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The accelerator pedal position (APP) sensors 1 and 2 are located within the accelerator pedal assembly. Each sensor has the following circuits:

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

The APP sensor provides the engine control module (ECM) with a signal voltage proportional to accelerator pedal movement. The APP sensor 1 signal voltage at rest position is near the low reference and increases as the pedal is actuated. The APP sensor 2 signal voltage at rest position is also near the low reference and increases as the pedal is actuated. When the ECM detects that the APP sensor 1 signal voltage is outside the predicted range, DTC P2120 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P2120 Accelerator Pedal Position (APP) Sensor 1 Circuit

Conditions for Running the DTC

    • The ignition is ON.
    • The ignition voltage is more than 5.25 volts.
    • DTC P0651 is not set.
    • DTC P2120 runs continuously when the above conditions are met.

Conditions for Setting the DTC

The APP sensor 1 voltage is less than 0.13 volts or more than 4.87 volts for more than 0.5 second.

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 and/or the Failure Records.
    • The control module commands the TAC system to operate in the Reduced Engine Power mode.
    • A message center or an indicator displays Reduced Engine Power.
    • Under certain conditions the control module commands the engine OFF.

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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Engine Control Module Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the accelerator pedal position (APP) sensor 1 voltage with the accelerator pedal in the rest position with a scan tool.

Does the scan tool indicate voltage less than the first value or greater than the second value?

0.13 V

4.87 V

Go to Step 5

Go to Step 3

3

Is DTC P2138 also set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 4

4

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 90 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.

Does the DTC fail this ignition?

--

Go to Step 5

Go to Testing for Intermittent Conditions and Poor Connections

5

  1. Turn OFF the ignition.
  2. Disconnect the accelerator pedal harness connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the APP sensor 1 voltage parameter with a scan tool.

Does the scan tool indicate that the voltage is less than the specified value?

0.13 V

Go to Step 6

Go to Step 11

6

  1. Connect a fused jumper wire between the APP sensor 1 5-volt reference circuit and the APP sensor 1 signal circuit at the accelerator pedal harness connector.
  2. Observe the APP sensor 1 voltage parameter with a scan tool.

Does the scan tool indicate that the APP sensor 1 voltage is more than the specified value?

4.8 V

Go to Step 7

Go to Step 8

7

  1. Turn OFF the ignition for 90 seconds.
  2. Allow the engine control module (ECM) to completely power down. This can be verified by the loss of communication on the scan tool.
  3. Measure the resistance from the low reference circuit of the APP sensor to the ECM case with the DMM.

Is the resistance less than the specified value?

5 ohms

Go to Step 17

Go to Step 13

8

Measure the voltage of the APP sensor 1 5-volt reference circuit with a DMM.

Does the DMM indicate that the voltage is more than the specified value?

4.8 V

Go to Step 10

Go to Step 9

9

Does the DMM indicate that the voltage is less than the specified value on the APP sensor 1 5-volt reference circuit?

5 V

Go to Step 14

Go to Step 16

10

Test the APP sensor 1 signal circuit for an open or for high resistance. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 12

11

Test the APP sensor 1 signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 19

12

Test the APP sensor 1 signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 19

13

Test the APP sensor 1 low reference circuit for an open or for high resistance. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 19

14

Test the APP sensor 1 5-volt reference circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 15

15

Test the APP sensor 1 5-volt reference circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 19

16

Test the APP sensor 1 5-volt reference circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 21

--

17

Inspect for poor connections at the accelerator pedal harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 18

18

Replace the accelerator pedal assembly. Refer to Accelerator Pedal with Position Sensor Assembly Replacement .

Did you complete the replacement?

--

Go to Step 21

--

19

Inspect for poor connections at the ECM harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 21

Go to Step 20

20

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

Did you complete the replacement?

--

Go to Step 21

--

21

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 90 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 22

22

Important: Be aware that repairing one individual condition may correct more than one DTC.

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK