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	Open/High Resistance	Short to Voltage	Signal Performance
APP Sensor 1 Signal	P2122	P2122	P2123	--
APP Sensor 1 5-Volt Reference	P0651	P2122	P0651	--
APP Sensor 1 Low Reference	--	P2123	--	--
APP Sensor 2 Signal	P2127	P2127	P2128	--
APP Sensor 2 5-Volt Reference	P0452, P0522, P0532, P0641	P2127	P0108, P0522, P0533, P0641	--
APP Sensor 2 Low Reference	--	P2128	--	--

Typical Scan Tool Data

APP Sensor 1

Circuit	Short to Ground	Open	Short to Voltage
Operating Conditions: The engine is operating at idle Parameter Normal Range APP Sensor 1: 0.33-4.59 V
5-Volt Reference 1	0 V	0 V	5 V
APP Sensor 1 Signal	0 V	0 V	5 V
APP Sensor 1 Low Reference	--	4.96 V	--

APP Sensor 2

Circuit	Short to Ground	Open	Short to Voltage
Operating Conditions: The engine is operating at idle Parameter Normal Range APP Sensor 2: 0.16-2.30 V
5-Volt Reference 2	0 V	0 V	5 V
APP Sensor 2 Signal	0 V	0 V	5 V
APP Sensor 2 Low Reference	--	4.92 V	--

Circuit/System Description

The throttle actuator control (TAC) system uses two 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 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 and monitor 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 ECM detects that APP sensor 1 voltage is less than 0.33 volt or greater than 4.59 volts for greater than 0.5 second.

P2122

The ECM detects that APP sensor 1 voltage is less than 0.33 volt for greater than 0.5 second.

P2123

The ECM detects that APP sensor 1 voltage is greater than 4.59 volts for greater than 0.5 second.

P2125

The ECM detects that APP sensor 2 voltage is less than 0.16 volt or greater than 2.30 volts for greater than 0.5 second.

P2127

The ECM detects that APP sensor 2 voltage is less than 0.16 volt for greater than 0.5 second.

P2128

The ECM detects that APP sensor 2 voltage is greater than 2.30 volts for greater than 0.5 second.

P2138

The ECM detects that the difference between APP sensor 1 and APP sensor 2 exceeds a predetermined value for greater than 0.5 second.

Action Taken When the DTC Sets

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

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 0.33-4.59 volts, and change with the accelerator pedal input.
2. Ignition ON, observe the scan tool APP sensor 2 voltage parameter. The reading should be between 0.16-2.30 volts, and change with the 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. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
6. Verify that the following DTCs are not set: DTC 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 DTCs P0641 or P0651 are set, refer to DTC P0641 or P0651.

Circuit/System Testing

1. Ignition OFF for 90 seconds, disconnect the harness connector at the accelerator pedal.
2. Test for less than 5 ohms 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.

3. 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 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 affected 5-volt reference circuit for short to voltage. If the circuit tests normal, replace the ECM.

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 B and E 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 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.

⇒	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.

6. 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 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.

7. Ignition OFF for 90 seconds, disconnect the harness connector at the ECM.
8. Test for less than 5 ohms on all APP sensor circuits between the following terminals:

•	ECM X1 low reference circuit terminal 37 to APP connector terminal A

•	ECM X1 low reference circuit terminal 36 to APP connector terminal D

•	ECM X1 signal circuit terminal 47 to APP connector terminal E

•	ECM X1 signal circuit terminal 49 to APP connector terminal B

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

•	ECM X1 5-volt reference circuit terminal 33 to APP connector 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.

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

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

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.