Diagnostic Instructions

DTC Descriptors

DTC P2120: Accelerator Pedal Position Sensor 1 Malfunction

DTC P2122: Accelerator Pedal Position Sensor 1 Voltage Low

DTC P2123: Accelerator Pedal Position Sensor 1 Voltage High

DTC P2125: Accelerator Pedal Position Sensor 2 Malfunction

DTC P2127: Accelerator Pedal Position Sensor 2 Voltage Low

DTC P2128: Accelerator Pedal Position Sensor 2 Voltage High

DTC P2138: Accelerator Pedal Position 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	P2127	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 and 2

Circuit	Normal Range	Short to Ground	Open or High Resistance	Short to Voltage
APP Sensor 1 Signal	0.33-4.75 V	0 V	0 V	5 V
APP Sensor 1 5-Volt Reference	0.33-4.75 V	0 V	0 V	5 V
APP Sensor 1 Low Reference	0.33-4.75 V	--	4.96 V	--
APP Sensor 2 Signal	0.33-2.50 V	0 V	0 V	5 V
APP Sensor 2 5-Volt Reference	0.33-2.50 V	0 V	0 V	5 V
APP Sensor 2 low Reference	0.33-2.50 V	--	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 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 .33 volt, or more than 4.59 volts for more than 2 seconds.

P2122

The ECM detects that the APP sensor 1 voltage is less than 0.33 volt for more than 2 seconds.

P2123

The ECM detects that the APP sensor 1 voltage is more than 4.59 volts for more than 2 seconds.

P2125

The APP sensor 2 voltage is less than 0.33 volt, or more than 4.59 volts for more than 2 seconds.

P2127

The ECM detects that the APP sensor 2 voltage is less than 0.33 volt for more than 2 seconds.

P2128

The ECM detects that the APP sensor 2 voltage is more than 4.59 volts for more than 2 seconds.

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

DTCs P2120, P2122, P2123, P2125, P2127, P2128 or P2138 are type A store only DTC's.

Conditions for Clearing the MIL/DTC

DTCs P2120, P2122, P2123, P2125, P2127, P2128 or P2138 are type A store only DTC's.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

Scan Tool Reference

Control Module References

Circuit/System Verification

1. Ignition ON, engine OFF, observe the DTC information on the scan tool.

⇒	If DTC P0641 or P0651 is also set diagnose those DTCs first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle .

2. With the accelerator pedal in the rest position, observe the APP Sensor 1 parameter with the scan tool. The voltage should be between 0.33-4.75 volts.

⇒	If the voltage is outside of the range continue with Circuit/System Testing.

3. Observe the APP Sensor 2 parameter with the scan tool. The voltage should be between 0.33-2.30 volts.

⇒	If the voltage is outside of the range continue with Circuit/System Testing.

4. Observe the APP Sensor 1 and 2 parameter with the scan tool. The sensors should indicate Agree.

⇒	If the sensors indicate Disagree, continue with Circuit/System Testing.

5. Clear the DTCs with a scan tool.
6. Turn OFF the ignition for 60 seconds.
7. Operate the vehicle within the Conditions for Running the DTCs.
8. Ignition ON, engine OFF, observe the previous APP sensor parameters and DTC information with the scan tool.

⇒	If DTC P2120 or P2125 is set and all of the previous APP sensor parameters are normal, replace the ECM.

⇒	If any of the previous APP sensor parameters are now abnormal or out of range continue with Circuit/System Testing.

Circuit/System Testing

1. Ignition OFF.
2. Disconnect the accelerator pedal harness connector.
3. Ignition ON, engine OFF, observe the APP Sensor 1 and APP Sensor 2 parameters on the scan tool. The voltage should be less than 0.1 volt.

⇒	If the voltage is more than 0.1 volt, test the affected signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

4. Measure for 4.8-5.2 volts between each of the 5 volt reference circuits and a known ground.

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

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

5. Instal a 3-A fusible link wire between the signal circuit and the 5 volt reference circuit of the APP sensor 1. Observe the APP Sensor 1 parameter on the scan tool. The voltage should be more than 4.8 volts.

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

6. Install a 3-A fusible link wire between the signal circuit and the 5 volt reference circuit of the APP sensor 2. Observe the APP Sensor 2 parameter on the scan tool. The voltage should be more than 4.8 volts.

⇒	If the voltage is less than 4.8 volts, test the APP sensor 2 signal circuit short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

7. Measure for 4.8-5.2 volts between one of the 5 volt reference circuits and each of the low reference circuits.

⇒	If the voltage is less than 4.8 volts, test the affected low reference for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

8. Ignition OFF.
9. Disconnect the ECM harness connector and measure the 5 volt reference, the low reference and each of the signal circuits between the ECM and the accelerator pedal for less than 5 ohms.

⇒	If the resistance is more than 5 ohms, repair the affected circuit.

10. Test the signal circuits for continuity between them.

⇒	If continuity exists between the signal circuits, repair the short.

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

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.