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
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: Ignition ON or Engine running. Parameter Normal Range: APP 1 varies 0.90-4.35 V
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: Ignition ON or Engine running. Parameter Normal Range: APP 2 varies 0.43-2.15 V
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 throttle actuator control (TAC) system uses two accelerator pedal position (APP) sensors to monitor the accelerator pedal position. The sensors 1 and 2 are located within the pedal control assembly. Each sensor has a 5-volt reference circuit, a low reference circuit, and a signal circuit.

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. The processors share the monitor data to verify that the indicated APP calculation is correct.

Conditions for Running DTC

P2120, P2122, P2123, P2125, P2127, P2128

P2138

Conditions for Setting DTC

P2120

The ECM detects that the APP sensor 1 voltage is less than 0.33 volt or greater than 4.5 volts for less than 1 second.

P2122

The ECM detects that the APP sensor 1 voltage is less than 0.33 volt for less than 1 second.

P2123

The ECM detects that the APP sensor 1 voltage is more than 4.5 volts for less than 1 second.

P2125

The ECM detects that the APP sensor 2 voltage is less than 0.33 volt or greater than 4.5 volts for less than 1 second.

P2127

The ECM detects that the APP sensor 2 voltage is less than or equal to 0.33 volt for less than 1 second.

P2128

The ECM detects that the APP sensor 2 voltage is greater than 4.5 volts for less than 1 second.

P2138

The voltage difference between APP sensor 1 and APP sensor 2 exceeds a value of 5 percent with the pedal in the released position, or 10 percent with the pedal at wide open throttle (WOT) for less than 1 second.

Action Taken When the DTC Sets

Conditions for Clearing the MIL/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

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 0.33-4.5 volts and change with accelerator pedal input.
2. Ignition ON, observe the scan tool APP Sensor 2 Voltage parameter. The reading should be between 0.33-4.5 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.
5. Operate the vehicle within the Conditions for Running the DTC or within the conditions that you observed from the Freeze Frame/Failure Records.
6. Verify that the following DTCs are not set: P0641, P0651, P2120, and P2125

⇒	If either or both DTCs P2120 or P2125 are set and they are the only DTCs set, replace the ECM.

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

Circuit/System Testing

Note: The first two steps of this diagnostic are necessary for an accurate low reference test result.

1. Ignition OFF, remove the Driver Dr. Mod. fuse from the left rear fuse block.
2. All access doors closed and ignition OFF for 90 seconds, access the vehicle from the driver's door while ensuring all other doors remain closed.
3. Disconnect the harness connector at the accelerator pedal.
4. Test for less than 5 ohms of resistance between each low reference circuit terminals A and D and ground.

⇒	If greater than 5 ohms, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

5. Install the Drivers Dr. Mod. fuse.
6. Ignition ON, test for 4.8-5.2 volts between each 5-volt reference circuit terminals C and F and ground.

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

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

7. 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 appropriate APP sensor 1 or 2 signal circuit terminal B or E for a short to voltage. If the circuit tests normal, replace the ECM.

8. Install a 3A fused jumper wire between the signal circuit terminal E and the 5-volt reference circuit terminal F of the APP sensor 1. Verify the scan tool parameter 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 ECM.

9. Install a 3A fused jumper wire between the signal circuit terminal B and the 5-volt reference circuit terminal C of the APP sensor 2. Verify that the scan tool parameter 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 ECM.

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

•	The ECM X1 signal circuit terminal 47 to the APP terminal E

•	The ECM X1 signal circuit terminal 49 to the APP terminal B

•	The ECM X1 5-volt reference circuit terminal 35 to the APP terminal C

•	The ECM X1 5-volt reference circuit terminal 33 to the APP terminal F

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

12. Test for infinite resistance between APP sensor 1 signal circuit terminal E and APP sensor 2 signal circuit terminal B.

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

13. If all circuits test normal, test or replace the accelerator pedal.

Component Test

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 0.40-4.50 volts with out an 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.