How To Test The GM 2.4L Throttle Position Sensor (TPS)

How To Test The GM 2.4L Throttle Position Sensor (TPS)

Troubleshooting and testing the throttle position sensor (TPS) or diagnostic trouble codes: P0121, P0122, P0123 on your 2.4L Quad 4 GM vehicle can be done on the car and with a simple multimeter. You won't need a scan tool to follow the steps in this article.

All of the three tests in this article are explained in a step-by-step manner and at the end of each test, you'll get a detailed interpretation of the possible results of the test.

In Spanish You can find this tutorial in Spanish here: Cómo Probar El Sensor TPS Con Multímetro (GM 2.4L) (at:

NOTE: To see all GM 2.4L (Quad 4) Test Articles, go here: GM 2.4L Index Of Articles.

Symptoms Of A Bad TPS

You'll have the check engine light on, for sure, on your instrument cluster and one of several of the following symptoms:

  • TPS diagnostic trouble codes (DTCs) stored in the vehicle's computer's memory.
  • Really bad gas mileage. You know that it's not the price of gasoline that has you thinking that your pick up or SUV is costing you more at the pump.
  • Transmission does not shift out of second gear. Now, this doesn't happen very often, but it happens.
  • No power and/or hesitation as you accelerate the vehicle. It feels like all of a sudden someone cut the power out momentarily as you step on the gas to get the vehicle moving.

How The Throttle Position Sensor Works

The throttle position sensor's job is to measure the angle of the throttle. So here, in a nutshell, is how the throttle position sensor works when you crank and start your 2.4L (Quad 4) GM equipped vehicle vehicle:

  1. The fuel injection computer supplies 5 Volts and Ground to the throttle position sensor.
  2. Now, since the throttle is closed, the TPS (with power and Ground supplied) sends the PCM (Powertrain Control Module=Fuel Injection Computer) a DC voltage signal of about 0.9 to 1 Volt. This value is what the PCM associates with a closed throttle.
  3. Once you throw the car in drive and accelerate the car, the throttle opens and the throttle position sensor immediately sends this change of the throttle angle as an INCREASING voltage signal to the PCM.
  4. With this increasing voltage signal, the PCM knows it's time to inject more fuel, advance ignition timing, and a host of other things it has to do to keep your 2.4L (Quad 4) GM equipped vehicle running optimally.
  5. As you let go off the accelerator pedal to slow down, the throttle plate closes and of course the TP sensor sends the info to the PCM as it returns to its base voltage signal, till the whole cycle begins again.

Pretty easy stuff? The cool thing is that the tests to check out the TP sensor's performance are as easy too. Now, since you'll be working in the engine compartment take all necessary safety precautions and use common sense. OK, enough of my yakking, let's get this show on the road.

TPS TEST 1: Testing The TPS Signal

Testing The TPS Signal With A Multimeter. How To Test The GM 2.4L Throttle Position Sensor (TPS)

This is a very easy and fast test (you'll spend about 20 minutes or so doing all three tests) but before you start, I recommend you read the entire article first, especially read the section: How The Throttle Position Sensor Works.

OK, let's jump into this first test. By the way, you'll need someone to help you complete this test step. Alright, this is what you'll need to do:


  1. 1

    Warm up the engine for about 20 minutes. This throttle position sensor test will be more effective and accurate if the engine is warmed up or at normal operating temperature.

  2. 2

    Grab your multimeter and select Volts DC mode. Now, with the red multimeter test lead, and using an appropriate tool (like a wire-piercing probe), probe the dark blue (DK BLU) wire of the throttle position sensor. This will be the wire that connects to the terminal labeled with the letter C in the illustration above.

    IMPORTANT: The throttle position sensor, on your 2.4L Quad 4 engine, has to remain connected to its electrical connector for this test.

  3. 3

    Ground the black multimeter test lead on the battery negative (-) terminal and then have your helper turn the key ON but don't start the engine.

  4. 4

    Your multimeter, if all is OK with the TPS, should register 0.5 to 0.7 Volts DC. If your multimeter registers something else, don't worry about it just yet, continue with the other steps.


  1. 5

    With the multimeter still connected to the dark blue (DK BLU) wire and the black multimeter test lead Grounded and the key in the ON position, rotate the throttle plate by hand while you observe the multimeter.

  2. 6

    The voltage the multimeter registered in test step 4 should increase smoothly as you manually open the throttle plate to its Wide Open Position. At Wide Open Throttle, your multimeter should be registering about 4.5 to 4.9 Volts DC

  3. 7

    Now, release the throttle plate to its original closed position (do this slowly). Again, keep observing the voltage readings on your multimeter. The voltage should decrease in a smooth and linear fashion back to the exact same voltage you noticed in step 4.


  1. 8

    OK, now your helper needs to help you with the next steps. Have him or her, lightly tap the TP sensor with the butt of a screw-driver's handle (or something similar, and I want to emphasize the words 'lightly tap').

  2. 9

    As your helper taps the throttle position sensor with the screw-driver, you need to open and close the throttle (smoothly and slowly of course).

    What you are trying to confirm with this, is that the tapping of the screw-driver should have not effect on the voltage readings your multimeter is recording. In other words, there should not be any gaps or skips in the readings as your helper taps the TPS.

  3. 10

    Repeat steps 8 and 9 several times to make sure of your multimeter test results.

Let's find out what your test results mean:

CASE 1: The multimeter registered a smooth increase or decrease in voltage. This is the correct test result.

You can conclude that the throttle position sensor has passed with flying colors. No further tests are needed, since these test results confirm that the TPS is good.

Now, if the throttle position sensor code keeps coming back, take a look at the heading: TPS Code Will Not Go Away for a few more suggestions as to what could be causing the TPS diagnostic trouble code (DTC).

CASE 2: The multimeter's voltage reading showed gaps when tapping the TPS with the screwdriver. This means that the throttle position sensor (TPS) is bad. Replacing the throttle position sensor will solve the TPS diagnostic trouble code (DTC) lighting up your check engine light (CEL).

CASE 3: The multimeter DID NOT register any voltage. This is not good, but does not condemn the TPS as bad yet. Two more things need to be verified first, which are making sure the TPS sensor is getting 5 Volts and Ground. Go to: TPS TEST 2: Testing The 5 Volt Reference Signal.

Buick Vehicles:

  • Skylark 2.4L
    • 1997, 1998, 1999

Chevrolet Vehicles:

  • Cavalier 2.4L
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Malibu 2.4L
    • 1997, 1998, 1999

Oldsmobile Vehicles:

  • Achieva 2.4L
    • 1996, 1997, 1998
  • Alero 2.4L
    • 1999, 2000, 2001

Pontiac Vehicles:

  • Grand Am 2.4L
    • 1996, 1997, 1998, 1999, 2000, 2001

Pontiac Vehicles:

  • Sunfire 2.4L
    • 1996, 1997, 1998, 1999, 2000, 2001