GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2009 🢒 SLS 🢒 Engine 🢒 Engine Controls and Fuel - 4.6L (LH2) 🢒 DTC P2120, P2122, P2123, P2125, P2127, P2128, or P2138

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P2120: Accelerator Pedal Position (APP) Sensor 1 Circuit

DTC P2122: Accelerator Pedal Position (APP) Sensor 1 Circuit Low Voltage

DTC P2123: Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage

DTC P2125: Accelerator Pedal Position (APP) Sensor 2 Circuit

DTC P2127: Accelerator Pedal Position (APP) Sensor 2 Circuit Low Voltage

DTC P2128: Accelerator Pedal Position (APP) Sensor 2 Circuit High Voltage

DTC P2138: Accelerator Pedal Position (APP) Sensor 1-2 Correlation

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

APP Sensor 1 Signal

P2122

P2138

P2122

P2123

APP sensor 1 5-Volt Reference

P2122, P0651

P2138

P2122

P2123, P0651

APP Sensor 1 Low Reference

--

P2138

P2123

--

APP Sensor 2 Signal

P2127

P2138

P2127

P2128

APP sensor 2 5-Volt Reference

P2127, P0641

P2138

P2127

P2128, P0641

APP Sensor 2 Low Reference

--

P2138

P2128

--

Typical Scan Tool Data

APP Sensor 1

Circuit

Short to Ground

Open

Short to Voltage

Operating Conditions: The ignition is ON or the engine is operating.

Parameter Normal Range: Accelerator Pedal Position 1 changes from 0.90 to 4.35 volts

APP Sensor 1 Signal

0 V

0 V

4.98 V

APP Sensor 1 5-Volt Reference

0 V

0 V

2.50-4.98 V

APP Sensor 1 Low Reference

--

4.95 V

--


APP Sensor 2

Circuit

Short to Ground

Open

Short to Voltage

Operating Conditions: The ignition is ON or the engine is operating.

Parameter Normal Range: Accelerator Pedal Position 2 changes between 0.43 and 2.15 volts

APP Sensor 2 Signal

0 V

0 V

4.98 V

APP sensor 2 5-Volt Reference

0 V

0 V

1.18-1.25 V

APP Sensor 2 Low Reference

--

4.90 V

--

Circuit/System Description

The accelerator pedal position (APP) sensor is mounted on the accelerator pedal control assembly. Two APP sensors, APP sensors 1 and 2 are used to monitor the accelerator pedal position.

    • The 5-volt reference circuit
    • Low reference circuit
    • The signal circuit

The Engine Control Module (ECM) uses the APP sensors input to determine the desired amount of acceleration or deceleration.

Conditions for Running the DTC
    • DTC P0641 or P0651 are not set.
    • The ignition switch is in Unlock/Accessory, Run or Ignition position.
    • The battery voltage is greater than 5.23 volts.
    • DTC P2120, P2122, P2123, P2125, P2127, P2128 or P2138 runs continuously when the above conditions are met.

Conditions for Setting the DTC

P2120

    • The ECM detects that the APP sensor 1 voltage is less than 0.33 volt or more than 4.8 volt.
    • It occurs 19 times or 13 times continuously during 39 tests

P2122

    • The ECM detects that the APP sensor 1 voltage is less than 0.33 volt.
    • It occurs 19 times or 13 times continuously during the 39 tests.

P2123

    • The ECM detects that the APP sensor 1 voltage is more than 0.33 volt.
    • It occurs 19 times or 13 times continuously during the 39 tests.

P2125

    • The ECM detects that the APP sensor 2 voltage is less than 0.33 volt or more than 4.8 volt.
    • It occurs 19 times or 13 times continuously during the 39 tests.

P2127

    • The ECM detects that the APP sensor 2 voltage is less than 0.33 volt.
    • It occurs 19 times or 13 times continuously during the 39 tests.

P2128

    • The ECM detects that the APP sensor 2 voltage is more than 4.8 volt.
    • It occurs 19 times or 13 times continuously during the 39 tests.

P2138

    • The voltage difference between APP sensor 1 and APP sensor 2 exceeds 5 percent of the value with the pedal in the released position, or 10 percent with the pedal at Wide Open Throttle (WOT).
    • It occurs 19 times or 15 times continuously during the 39 tests.

Action Taken When the DTC Sets
    • DTCs P2120, P2122, P2123, P2125, P2127, P2128, and P2138 are Type A DTCs.
    • The control module commands the TAC system to operate in the Reduced Engine Power mode.
    • The message center or the indicator displays Reduced Engine Power.
    • Under certain conditions the control module commands the engine OFF.

Conditions for Clearing the DTC

DTCs P2120, P2122, P2123, P2125, P2127, P2128, and P2138 are Type A DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Throttle Actuator Control (TAC) System Description

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions

Circuit/System Verification
  1. Ignition ON, observe the scan tool APP sensor 1 voltage parameter. The reading should be between 0.33-4.5 volts and changes with accelerator pedal input.
  2. Ignition ON, observe the scan tool APP sensor 2 voltage parameter. The reading should be between 0.33-4.5 volts and changes with accelerator pedal input.
  3. Ignition ON, observe the scan tool APP sensor 1 and 2 parameter. The scan tool should indicate Agree.
  4. Clear the DTCs with the scan tool. Operate the vehicle within the Conditions for Running the DTC, or within the conditions that you observed from the Freeze Frame/Failure Records.
    5. ⇒If only DTC P2120 or P2125 is set, replace the control module.
    ⇒If DTC P0641 or P0651 is set, refer to DTC P0641 or P0651.

Circuit/System Testing
  1. Ignition OFF, disconnect the harness connector at the accelerator pedal. Allow the ECM to remove power for at least 60 seconds.
  2. Test for less than 5 ohms between the low reference circuit terminal A and D and ground.
    3. ⇒If more than 5 ohms, test the low reference circuit for an open/high resistance. If the circuit test normal, replace the ECM.
  3. Ignition ON, test for 4.8-5.2 volts between the 5-volt reference circuit terminal C and F and ground.
    4. ⇒If less than 4.8 volts, test the 5-volt reference circuit for an open, high resistance or short to ground. If the circuit test normal, replace the ECM.
    ⇒If greater than 5.2 volts, test the affected 5-volt reference circuit for short to voltage. If the circuit test normal, replace the ECM.
  4. Ignition ON, verify that the scan tool APP sensor 1 and 2 voltages are less than 0.1 volt.
    5. ⇒If greater than 0.1 volt, test the APP sensor 1 and 2 signal circuits terminals B and E for a short to voltage. If the circuit test normal, replace the ECM.
  5. Install a 3-amp fused jumper wire between the signal circuit terminal E and the 5-volt reference circuit terminal F of the APP sensor 1. Verify the APP sensor 1 voltage is greater than 4.8 volts.
    6. ⇒If less than 4.8 volts, test the APP sensor 2 signal circuit for an open, high excessive resistance or short to ground. If the circuit test normal, replace the ECM.
  6. Install a 3-amp fused jumper wire between the signal circuit terminal B and the 5-volt reference voltage circuit terminal C of the APP sensor 2. Verify the APP sensor 2 voltage is greater than 4.8 volts.
    7. ⇒If less than 4.8 volts, test the APP sensor 2 signal circuit for an open, high resistance or short to ground. If the circuit tests normal, replace the ECM.
  7. Ignition OFF, disconnect the harness connector at the ECM.
  8. Test for less than 5 ohms on all APP sensor circuits between the following terminals:
    9. • The ECM C1 signal circuit terminal 47 to the APP terminal E
    • The ECM C1 signal circuit terminal 49 to the APP terminal B
    • The ECM C1 5-volt reference circuit terminal 35 to the APP terminal C
    • The ECM C1 5-volt reference circuit terminal 33 to the APP terminal F
    ⇒If greater than 5 ohms, repair the affected circuit.
  9. Test for infinite resistance between APP sensor 1 signal circuit terminal E and APP sensor 2 signal circuit terminal B.
    10. ⇒If not the infinite, repair the short condition between APP sensor 1 signal circuit and APP sensor 2 signal circuit.
    ⇒If all circuits test normal, replace the accelerator pedal.

Component Testing

Dynamic Test

  1. Install a 3-amp fused jumper wire between the 5-volt reference circuit terminal of the appropriate APP sensor and 5-volt reference. Install a jumper wire between the low reference terminal and a ground.
  2. Sweep the sensor through the entire range while monitoring the voltage between the signal terminal and the low reference terminal with a DMM. The voltage should vary within 0.40-4.50 volts with out an spikes or dropouts.
    3. ⇒If the voltage is not within the specified range or is abnormal, replace the accelerator pedal assembly.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Accelerator Pedal Position Sensor Replacement
    •  Control Module References for ECM replacement, setup, and programming