Diagnostic Instructions

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	High Resistance	Open	Short to Voltage	Signal Performance
APP Sensor 1 Signal	P2122	P2138	P2122	P2123	--
APP sensor 1 5-Volt Reference	P2122	P2138	P2122	P2123	--
APP sensor 1 Low Reference	--	P2138	P2123	--	--
APP Sensor 2 Signal	P2127	P2138	P2128	P2128	--
APP sensor 2 5-Volt Reference	P2127	P2138	P2127	P2128	--
APP sensor 2 Low Reference	--	P2138	P2128	--	--

Typical Scan Tool Data

APP Sensor 1

Circuit	Short to Ground	Open	Short to Voltage
Parameter Normal Range: 0.32-4.75 V
APP Sensor 1 Signal	0 V	0 V	5 V
APP Sensor 1 5-Volt Reference	0 V	0 V	5 V
APP Sensor 1 Low Reference	--	5 V	--

APP Sensor 2

Circuit	Short to Ground	Open	Short to Voltage
Parameter Normal Range: 0.32-4.75 V
APP Sensor 2 Signal	0 V	0 V	5 V
APP Sensor 2 5-Volt Reference	0 V	0 V	5 V
APP Sensor 2 Low Reference	--	5 V	--

Circuit/System Description

The throttle actuator control (TAC) system uses 2 accelerator pedal position (APP) sensors to monitor the accelerator pedal position. The APP sensors 1 and 2 are located within the pedal assembly. Each sensor has the following circuits:

Two processors are also used to monitor the TAC system data. Both processors are located within the engine control module (ECM). Each signal circuit provides both processors with a signal voltage proportional to pedal movement. Both processors monitor each other's data to verify that the indicated APP calculation is correct.

Conditions for Running the DTC

P2120, P2122, P2123, P2125, P2127, P2128

P2138

Conditions for Setting the DTC

P2120

The APP sensor 1 voltage is less than 0.32 volt, or more than 4.75 volts for more than 0.4 second.

P2122

The ECM detects that the APP sensor 1 voltage is less than 0.32 volt for more than 0.4 second.

P2123

The ECM detects that the APP sensor 1 voltage is more than 4.75 volts for more than 0.4 second.

P2125

The APP sensor 2 voltage is less than 0.32 volt, or more than 4.75 volts for more than 0.4 second.

P2127

The ECM detects that the APP sensor 2 voltage is less than 0.32 volt for more than 0.4 second.

P2128

The ECM detects that the APP sensor 2 voltage is more than 4.75 volts for more than 0.4 second.

P2138

The voltage difference between APP sensor 1 and APP sensor 2 exceeds a predetermined value for more than 2 seconds.

Action Taken When the DTC Sets

Conditions for Clearing the MIL/DTC

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

Reference Information

Schematic Reference

Engine Controls Schematics

Description and Operation

Throttle Actuator Control (TAC) System Description

Connector End View Reference

Component Connector End Views

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

1. Ignition ON, observe the scan tool APP sensor 1 voltage parameter. The reading should be between 0.32-4.75 volts and change with accelerator pedal input.
2. Ignition ON, observe the scan tool APP sensor 2 voltage parameter. The reading should be between 0.32-4.75 volts and change 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. Verify that DTC P2120 or P2125 are not the only accelerator pedal position DTCs set.

⇒	If DTC P2120 or P2125 are the only DTCs set, replace the control module.

6. Verify that DTCs P0641 or P0651 are set.

⇒	If DTC P0641 or P0651 are set, refer to DTC P0641 or P0651.

7. 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 data.

Circuit/System Testing

1. Ignition ON, disconnect the accelerator pedal harness connector at the accelerator pedal. Allow sufficient time for the ECM to power down.
2. Test for less than 3 ohms of resistance between each low reference circuit terminal 4 and 5 and ground.

⇒	If greater than 3 ohms, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

3. Ignition ON, test for 4.8-5.2 volts between each 5-volt reference circuit terminals 1 and 2 and ground.

⇒	If less than 4.8 volts, test affected 5-volt reference circuit for an open/high resistance or short to ground. If the circuit tests normal, replace the control module.

⇒	If greater than 5.2 volts, test affected 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the control module.

4. Ignition ON, verify the scan tool APP sensor 1 and 2 voltages are less than 0.1 volt.

⇒	If greater than 0.1 volt, test the APP sensor 1 and 2 signal circuits terminals 3 and 6 for a short to voltage. If the circuit tests normal, replace the ECM.

5. Install a 3A fused jumper wire between the signal circuit terminal 3 and the 5-volt reference circuit terminal 2 of the APP sensor 1, and verify the APP sensor 1 voltage is greater than 4.8 volts.

⇒	If less than 4.8 volts, test the APP sensor 1 signal circuit for an open, or short to ground. If the circuit tests normal, replace the control module.

6. Install a 3A fused jumper wire between the signal circuit terminal 6 and the 5-volt reference circuit terminal 1 of the APP sensor 2, and verify that the APP sensor 2 voltage is greater than 4.8 volts.

⇒	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 control module.

7. Ignition OFF, disconnect the harness connector at the ECM.
8. Test for less than 5 ohms of resistance on all APP sensor circuits between the following terminals:

•	The signal circuit terminal 3 to the corresponding ECM terminal

•	The signal circuit terminal 6 to the corresponding ECM terminal

•	The 5-volt reference circuit terminal 1 to the corresponding ECM terminal

•	The 5-volt reference circuit terminal 2 to the corresponding ECM terminal

⇒	If greater than 5 ohms, repair the affected circuit.

9. Test for infinite resistance between APP sensor 1 signal circuit terminal 3 and APP sensor 2 signal circuit terminal 6.

⇒	If less than infinite resistance, repair the short between APP sensor 1 signal circuit and APP sensor 2 signal circuit.

⇒	If all circuits test normal, replace the accelerator pedal.

Component Testing

Important: Circuit/System Testing must be performed before proceeding with Component Testing.

Dynamic Test

1. Install a 3A fused jumper wire between the 5-volt reference terminal of the applicable APP sensor and 5 volts. 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 between 0.32-4.75 volts without any spikes or dropouts.

⇒	If the voltage is not within the specified range or is erratic, replace the accelerator pedal assembly.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.