How To Test The GM Accelerator Pedal Position (APP) Sensor 2

TEST 2: 5 Volt Reference Circuit Test

Accelerator Pedal Position (APP) Sensor 2 needs power and Ground to produce a position signal. This power comes in the form of 5 Volts DC and are provided by the Throttle Actuator Control (TAC) Module.

In this second test, we're gonna' make sure that APP Sensor 2 is being fed with these 5 Volts, which means that you'll be testing for these 5 Volts DC at the APP sensor connector (in the vehicle).

The wire that you'll be testing is usually a Tan colored wire that feeds the pin labeled with the letter D of the APP Sensor Assembly.

IMPORTANT: The image, above, shows you the front of the connector just to make it easier to identify the circuit you need to test. When testing the circuit, you should back-probe the connector or probe the wire (with a wire-piercing probe), or you could damage the metal female terminal! This is a headache you definitely want to avoid.

Alright, this is what you need to do:

1. Locate the wire on the APP sensor connector (inside the vehicle) that belongs to circuit D.
   • You don't have to have the APP Sensor Assembly bolted in place (in the vehicle) or connected to its connector for this test. If you do, this test info still applies.
   • The APP Sensor Assembly connector has the letter D embossed on it, to further aid you in identifying it.
2. Place your multimeter in Volts DC mode.
3. With the red multimeter test lead, probe the wire identified with the letter D.
   • This is usually a Tan colored wire.
   • Note: If the wire is a different color, identify the wire by the letter that's embossed on the connector.
4. Ground the black multimeter test lead directly on the battery negative (-) terminal.
5. Have a helper turn the key ON, but don't crank or start the engine.
6. If all is OK, your multimeter should register about 5 Volts.
7. Let's interpret your test results below:

Let's take a look at your test results:

CASE 1: Your multimeter registered 5 Volts. So far so good, since this test result lets you know that Accelerator Pedal Position (APP) Sensor 2 is getting power.

The next step is to check that Accelerator Pedal Position (APP) Sensor 2 is getting Ground. Go to: TEST 3: Reference Low (Ground) Circuit Test.

CASE 2: Your multimeter DID NOT register 5 Volts. Double check your connections and make sure you're testing the correct wire.

If your multimeter still does not register 5 Volts in this circuit, then you've found the cause of the Accelerator Pedal Position (APP) Sensor 2 diagnostic trouble code (DTC). Repair the issue causing these missing 5 Volts, since without power APP Sensor 2 won't function.

Now, although it's beyond the scope of this tutorial to troubleshoot these missing 5 Volts, here are the two most like causes:

1. There's an open-circuit problem on this wire between the APP Sensor Assembly's connector and the Throttle Actuator Control (TAC) Module.
2. The Throttle Actuator Control (TAC) Module is bad.

TEST 3: Reference Low (Ground) Circuit Test

Now that you have confirmed that:

• Accelerator Pedal Position (APP) Sensor 2 passed the resistance test with flying colors (TEST 1).
•  -AND-
• APP Sensor 2 is getting power in the form of 5 Volts. (TEST 2).

The next step is to verify that Accelerator Pedal Position (APP) Sensor 2 is being fed with Ground. This Ground is provided to APP Sensor 2 thru' the Purple that belongs to circuit letter B (see photo above).

This Ground, like the 5 Volts you tested in the previous test, is provided by the Throttle Actuator Control (TAC) Module. This means that you need to be careful to NOT short this circuit to 12 Volts intentionally or accidentally or you're gonna' fry the TAC Module.

IMPORTANT: The image, above, shows you the front of the connector just to make it easier to identify the circuit you need to test. When testing the circuit, you should back-probe the connector or probe the wire (with a wire-piercing probe), or you could damage the metal female terminal! This is a headache you definitely want to avoid.

Alright, this is what you need to do:

1. Place your multimeter in Volts DC mode.
2. With the black multimeter test lead, probe the wire identified with the letter B in the photo above.
   • This is usually a Purple colored wire of the connector.
   • Note: If the wire is a different color, identify the wire by the letter that's embossed on the connector.
3. Connect the red multimeter test lead directly on the battery's positive (+) terminal using a battery jump start cable.
   • NOTE: The jumper cable will be 'live' with 12 Volts, so be careful and wrap the jumper cable end (that you've brought into the vehicle) in a shop towel or plastic bag. This will keep it from shorting to any metal surface.
4. Have a helper turn the key ON, but don't crank or start the engine.
5. If all is OK, your multimeter should register 10 to 12 Volts.
6. Let's interpret your test results below:

Let's take a look at what your test results mean:

CASE 1: Your multimeter registered 10 to 12 Volts. So far so good, since this test result lets you know that the Throttle Actuator Control (TAC) Module is providing Ground to Accelerator Pedal Position (APP) Sensor 2.

The next step is to check that APP Sensor 2 is creating the correct pedal position signal. Go to: TEST 4: Testing The APP Sensor 2 Signal Circuit.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts. Double check your connections and make sure you're testing the correct wire.

If your multimeter still does not register 10 to 12 Volts in this circuit, then you've found the cause of the APP Sensor 2 Diagnostic Trouble Code (DTC). Repairing the cause of this missing Ground will get APP Sensor 1 back to work, since without Ground, APP Sensor 2 won't work.

Now, although it's beyond the scope of this tutorial to troubleshoot the problem with this Ground, but here are the two most like causes:

1. There's an open-circuit problem on this wire between the APP Sensor Assembly's connector and the Throttle Actuator Control (TAC) Module.
2. The Throttle Actuator Control (TAC) Module is bad.

TEST 4: Testing The APP Sensor 2 Signal Circuit

So far in your tests have shown that:

• APP Sensor 2 has no internal problems/defects (TEST 1).
• APP Sensor 2 is getting juice, well 5 Volts DC. (TEST 2).
• APP Sensor 2 is getting Ground (TEST 3).

What we need to do now is to see if APP Sensor 2 is producing a voltage signal. This voltage signal starts high (around 4 Volts DC) when the pedal IS NOT being depressed and decreases (to around 2.9 Volts DC) as you depress the pedal.

NOTE: This test has to be done with the Accelerator Pedal Position Assembly back in its place (in the vehicle) and connected to its electrical connector.

OK, to get this cake baked, this is what you need to do:

1. If you haven't done so already, install the APP Sensor Assembly and bolt it down.
2. Reconnect the APP Sensor to its connector and reconnect the battery negative cable to the battery negative post.
3. Turn the key ON but don't crank or start the engine.
4. Set your multimeter in Volts DC mode.
5. With the APP Sensor Assembly connected to its connector, probe the Light Blue wire.
   • This is the wire identified with the letter C in the photo of the connector above.
   • This is where a wire piercing probe comes in handy since it'll let you easily access the wire inside the insulation (to see what this tool looks like, click here: Wire Piercing Tool).
6. Ground the multimeter's black test lead directly on the battery negative (-) terminal.
7. Observe your multimeter's voltage DC reading.
8. You should see about 3.8 Volts to 4.0L Volts DC.
9. Now, manually push down the accelerator pedal till it bottoms out.
   • When the pedal is fully depressed, you should see about 2.9 Volts DC.
   • NOTE: If all is OK, your multimeter should register a smooth voltage DECREASE with no gaps in the voltage numbers.
10. After pushing the Pedal all the way to the floor, slowly release it as you watch the numbers on your multimeter.
    • If all is OK, your multimeter should register a smooth voltage decrease with no gaps in the voltage numbers.

Let's examine your test results:

CASE 1: Your multimeter registered the indicated voltages with NO gaps. This is the correct and expected test result and lets you know that APP Sensor 2 is OK.

Now, if you have performed all of the tests (in this tutorial) and they have all passed with flying colors, YET you still have an APP Sensor 2 diagnostic trouble code, there's a good chance that:

1. There's an open-circuit problem on this wire between the APP Sensor Assembly's connector and the Throttle Actuator Control (TAC) Module.
2. The Throttle Actuator Control (TAC) Module is bad.

It's beyond the scope of this article to troubleshoot these specific issues, but you have at least eliminated APP Sensor 2 as being bad.

CASE 2: Your multimeter DID NOT register the indicated voltage and/or showed gaps in the voltage numbers. This is not good and tells you that APP Sensor 2 is malfunctioning.

You can confidently replace the APP Sensor Assembly if the APP Sensor Assembly failed this test and TEST 1.

Some Final Thoughts On APP Sensor 2

I remember when GM electronic throttle body equipped vehicles started rolling into the auto repair shop, where I worked, and feeling a bit nervous about having to diagnose them (to later realize just how easy it is to diagnose them!).

In a fully equipped auto repair shop, diagnosing an APP sensor problem is no big deal, but that's not the case when you're a DIY'er and the generic scan tool you own doesn't have any of the 'professional' features a $5K scan tool does.

And so, I want to assure you that the key, to successfully diagnosing an Accelerator Pedal Position (APP) sensor problem on your own, is remembering that the APP Sensor(s) inside the Accelerator Pedal Position Assembly are just potentiometers (like the old style throttle position sensors on fuel injection systems with a throttle cable).

Since they are just potentiometers, they can be resistance tested with a multimeter (like TEST 1 shows) or voltage tested (like TEST 4 showed).