GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2007 🢒 Isuzu MD F-Series 🢒 DTC 🢒 DTC P1278

Circuit Description

The accelerator pedal position (APP) sensor is mounted on the accelerator pedal control assembly. The sensor is made up of three individual sensors within one housing. The engine control module (ECM) uses the APP sensors to determine the amount of acceleration or deceleration desired by the person driving the vehicle via the fuel injector control. The APP sensor 1 has the following circuits:

    • A 5-volt reference circuit
    • A low reference circuit
    • APP sensor 1 signal circuit

The ECM supplies 5 volts to the APP sensor 1 on the 5-volt reference circuit. The ECM also provides a ground on the low reference circuit. The APP sensor 1 provides a signal to the ECM on the APP sensor 1 signal circuit which is relative to the position changes of the accelerator pedal angle. The ECM monitors the APP sensor 1 signals for voltage outside the normal range of the APP sensor 1. If the ECM detects an excessively high APP sensor 1 signal voltage, this DTC will set.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1278 Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage

Conditions for Running the DTC

The ignition switch is ON.

Conditions for Setting the DTC

The APP sensor 1 voltage is more than 4.75 volts.

Action Taken When the DTC Sets
    • The ECM does not illuminate the malfunction indicator lamp (MIL) when the diagnostic runs and tails.
    • The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.

Conditions for Clearing the DTC
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • Clear the DTC with a scan tool.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Turn OFF the ignition for 30 seconds.
  2. Start the engine.
  3. Observe the Accelerator Pedal Position (APP) Sensor 1 parameter with a scan tool.

Is the APP Sensor 1 parameter more than the specified value?

4.75 V

Go to Step 4

Go to Step 3

3

  1. Observe the Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Failure Records.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4
  1. Start the engine.
  2. Monitor the DTC Information with a scan tool.

Is DTC P0641 also set?

--

Go to Step 5

Go to Step 6

5
  1. Turn OFF the ignition.
  2. Disconnect the APP sensor connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the APP Sensor parameter with a scan tool.

Is the APP sensor 1 parameter less than the specified value?

0.25 V

Go to DTC P0641

Go to Step 8

6
  1. Turn OFF the ignition.
  2. Disconnect the APP sensor connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the APP Sensor 1 parameter with a scan tool.

Is the APP Sensor 1 parameter more than the specified value?

4.75 V

Go to Step 8

Go to Step 7

7
  1. Turn OFF the ignition.
  2. Disconnect the APP sensor connector.
  3. Connect a DMM between the 5-volt reference circuit and the low reference circuit of the APP sensor 1.
  4. Turn ON the ignition, with the engine OFF.

Is the DMM voltage within the specified range?

4.5-5.5 V

Go to Step 8

Go to Step 9

8

Important: The APP sensor 1 may be damaged if there is a circuit short to a voltage source.

  1. Test the signal circuit between the engine control module (ECM) and the APP sensor 1 for a short to voltage circuit.
  2. Repair the circuits as necessary.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 10

9
  1. Test the low reference circuit between the ECM and the APP sensor 1 for an open or high resistance.
  2. Repair the circuits as necessary.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

10
  1. Turn OFF the ignition.
  2. Disconnect the APP sensor connector.
  3. Inspect for poor connections at the harness connector of the APP sensor 1.
  4. Repair the connections as necessary.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

11
  1. Turn OFF the ignition.
  2. Disconnect the ECM.
  3. Inspect for poor connections at the harness connector of the ECM.
  4. Repair the connections as necessary.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 13

12

Replace the APP sensor. Refer to Accelerator Pedal Position Sensor Replacement .

Did you complete the replacement?

--

Go to Step 14

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13

Important: The replacement ECM must be programmed.

Replace the ECM. Refer to Engine Control Module Replacement/Restore Fuel Rate Data , and Fuel Injector Flow Rate Programming .

Did you complete the replacement?

--

Go to Step 14

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14
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 15

15

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK