How To Test APP Sensor 1 And APP Sensor 2 (2004, 2005, 2006, 2007 4.2L Chevrolet TrailBlazer And GMC Envoy)

Testing APP sensor 1 and APP sensor 2 on the 2004-2007 4.2L Chevrolet TrailBlazer or GMC Envoy is something that can be done easily with just a multimeter.

In this tutorial I will explain how to do it in a step-by-step way so that you can find out if the APP sensor assembly is defective or not.

NOTE: This tutorial applies to the following vehicles (since they all use the exact same accelerator pedal assembly):

  1. 2004, 2005, 2006, 2007 4.2L Chevrolet Trailblazer.
  2. 2004, 2005, 2006, 2007 4.2L GMC Envoy.

En Español You can find this tutorial in Spanish here: Cómo Probar El Sensor APP 1 y 2 (2004-2007 4.2L Chevrolet TrailBlazer) (at: autotecnico-online.com).

NOTE: The following TAC system tutorials might be useful:

  1. TAC System Wiring Diagram (2002-2005 4.2L Chevrolet TrailBlazer).
  2. TAC System Wiring Diagram (2006-2007 4.2L Chevrolet TrailBlazer).
  3. Electronic Throttle Body Tests (2002-2007 4.2L Chevrolet TrailBlazer).
  4. How To Test APP Sensor 1 And 2 (2002-2003 4.2L Chevrolet TrailBlazer).

Symptoms Of A Bad APP Sensor

As you're probably already aware, the accelerator pedal comes equipped with two accelerator pedal position (APP) sensors. These are APP sensor 1 and APP sensor 2.

On older non-TAC equipped vehicles, the accelerator pedal did not have any sensors attached to it and was connected via a mechanical cable to the throttle plate in the throttle body.

On the 2004-2007 4.2L Chevrolet TrailBlazer and GMC Envoy this is no longer the case (since it's not mechanically connected to the throttle plate).

When you step on or off the accelerator pedal, the two APP sensors inform the computer how much the pedal has traveled.

The computer then commands the throttle actuator motor, in the electronic throttle body, to open or close the throttle plate.

When one or both of the APP sensors fail, you're going to see one or more of the following symptoms:

  1. The check light engine light will be lit up by one of the following trouble codes:
    1. P2120: Accelerator Pedal Position (APP) Sensor 1 Circuit.
    2. P2122: Accelerator Pedal Position (APP) Sensor 1 Circuit Low Voltage.
    3. P2123: Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage.
    4. P2125: Accelerator Pedal Position (APP) Sensor 2 Circuit.
    5. P2127: Accelerator Pedal Position (APP) Sensor 2 Circuit Low Voltage.
    6. P2128: Accelerator Pedal Position (APP) Sensor 2 Circuit High Voltage.
    7. P2138: Accelerator Pedal Position (APP) Sensor 1 - 2 Correlation.
  2. Your Chevy TrailBlazer (GMC Envoy) will run in Reduced Engine Power mode.
  3. The engine starts and runs but when you try to accelerate your Chevy TrailBlazer (GMC Envoy), it accelerates very slowly.
  4. The engine starts and runs but the engine does not accelerate when you step on the accelerator pedal.
  5. The engine starts and runs but shuts down.

APP Sensor Terminal Pin Out Chart

All of the tests that we're gonna' perform in this tutorial are done on the male spade terminals of the connector on the APP sensor assembly itself.

The electrical connector that plugs into the APP sensor assembly has female terminals. In the table below you'll find a description of the wires that come out of it:

Pin Wire Color Description
A PPL Sensor Ground
B LT BLU APP Sensor 2 Signal
C TAN 5 Volts
D BRN Ground
E DK BLU APP sensor 1 Signal
F WHT/BLK Sensor Ground

The following wiring diagram of the TAC system will help you further identify the APP sensor circuits: TAC System Wiring Diagram (2002-2005 4.2L Chevrolet TrailBlazer).

TEST 1: APP Sensor 1 Resistance Tests (1 Of 2)

How To Test APP Sensor 1 And APP Sensor 2 (2004, 2005, 2006, 2007 4.2L Chevrolet TrailBlazer And GMC Envoy)

We're going to start off by testing accelerator pedal position (APP) sensor 1.

Specifically, we're going to make sure that continuity exists between terminal D and terminal F.

If your multimeter shows an open-circuit or a short-circuit problem between these two terminals then we can conclude that APP sensor 1 is defective.

NOTE: All tests in this tutorial are done on the male spade terminals of the connector on the APP sensor assembly itself.

NOTE: Connecting the multimeter test leads to the indicated terminals of the APP sensor assembly can be a challenge. I personally use jumper wires with alligator clips on both ends to connect my test leads to the terminals. I've also been known to remove the actuator pedal to test it on my workbench.

These are the test steps:

  1. 1

    Disconnect the APP sensor assembly from its electrical connector.

  2. 2

    Place your multimeter in Ohms mode.

  3. 3

    Connect one multimeter test lead to terminal D.

  4. 4

    Connect the other multimeter test lead to terminal F.

  5. 5

    Your multimeter should read continuity.

    At this point I usually get 3.5 K Ohms for the resistance reading from a good APP 1 sensor.

Let's examine your test results:

CASE 1: The multimeter registered continuity between terminals D and F. This is the correct test result.

Now the next step is APP sensor 1 test 2 of 2. For this test go to: TEST 2: APP Sensor 1 Resistance Tests (2 Of 2).

CASE 2: The multimeter indicated an open-circuit or a short-circuit between terminals D and F. This test result lets you know that APP sensor 1 is defective.

Replace the accelerator pedal assembly.