How To Test APP Sensor 1 And APP Sensor 2 (2002-2003 4.2L Chevrolet TrailBlazer And GMC Envoy)

In this tutorial I'm going to explain how to test accelerator pedal position (APP) sensor 1 and sensor 2.

Testing both of these APP sensors simply involves testing the resistance between two terminals as the pedal is depressed and released.

The cool thing is that you don't need any expensive diagnostic equipment for these tests, since I'm going to show you how to test them using only a multimeter.

You'll be able to find out if APP sensor 1 or APP sensor 2 is defective or not.

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

  1. 2002, 2003 4.2L Chevrolet Trailblazer.
  2. 2002, 2003 4.2L GMC Envoy.

En Español You can find this tutorial in Spanish here: Cómo Probar El Sensor APP 1 y 2 (2002-2003 4.2L Chevrolet TrailBlazer) (at: autotecnico-online.com).

NOTE: The following TAC system wiring diagram may come in handy: TAC System Wiring Diagram (2002-2005 4.2L Chevrolet TrailBlazer).

If you need to test the electronic throttle body, you can find the tutorial here: Electronic Throttle Body Tests (2002-2007 4.2L Chevrolet TrailBlazer).

Symptoms Of A Bad APP Sensor

In the old days, a mechanical accelerator cable connected the accelerator pedal to the throttle plate (in the throttle body).

In today's modern GM vehicles equipped with the throttle actuator control (TAC) system, the mechanical accelerator cable has been replaced by electronic components.

The APP sensor assembly is the component that lets your Chevy TrailBlazer or GMC Envoy's fuel injection computer know how much you have stepped on or off the accelerator pedal.

The fuel injection computer then uses this information to open the throttle plate a corresponding amount.

Since the APP sensor assembly is a critical component of the fuel injection system, when one of the APP sensors fail you are 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

Each APP sensor is a potentiometer type position sensor. In plain English this means that the sensor needs 5 Volts and Ground to be able to create a variable voltage signal.

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

To get a better idea of the APP sensor 1 and APP sensor 2 circuits, take a look at the following wiring diagram: TAC System Wiring Diagram (2002-2005 4.2L Chevrolet TrailBlazer).

Pin Wire Color Description
E DK BLU APP Sensor 1 Signal
D WHT/BLK 5 Volts
G PPL Sensor Ground
H LT BLU APP Sensor 2 Signal
J TAN 5 Volts
K BRN Sensor Ground

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

How To Test APP Sensor 1 And APP Sensor 2 (2002-2003 4.2L Chevrolet TrailBlazer And GMC Envoy)

We're going to start off our APP sensor assembly diagnostic by testing the resistance of terminal D and terminal K.

These two terminals belong to APP sensor 1. Terminal D is one that receives 5 Volts and terminal K is one that receives Ground from your Chevy Trailblazer or GMC Envoy's fuel injection computer.

The purpose of this test is to make sure that the circuit between these two terminals has continuity.

To be a bit more specific, we need to make sure that the circuit hasn't suffered a short-circuit problem or an open-circuit problem.

If the circuit has suffered a short circuit problem, your multimeter will give you a reading of less than 1 Ohm.

If the circuit has an open-circuit problem, your multimeter will not show continuity. If you're using a digital multimeter, this is usually represented by the letters OL (over-limit).

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

TIP: The terminals on the APP sensor assembly are not very accessible. I use jumper wires with alligator clips (on both ends) to connect my multimeter test leads to the correct terminals on the APP sensor assembly. I've also been known to remove the accelerator pedal to test it on my work table. I reccomend either option.

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 K.

  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 K. This is the correct test result.

With the first test of two out of the way, the next APP sensor 1 test is to make sure it produces the correct resistance values as you depress and release the pedal. 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 K. This lets you know that APP sensor 1 is defective within the accelerator pedal assembly.

Replace the accelerator pedal assembly.