How To Test The Throttle Position Sensor (GM 3.1L, 3.4L)

This article will take you step by step thru' the testing and troubleshooting of the throttle position sensor on your GM 3.1L or 3.4L V6 equipped car (Buick, Chevrolet, Oldsmobile, Pontiac). You don't need an automotive scan tool, since all of the tests, in this article, are done with just a simple multimeter.

Before you start the TPS tests, read this section first. At the bottom you'll find a complete list of GM makes and models this test info applies to. Also, here you'll find important ‘do's and don'ts’ that'll make testing the TP sensor on your car a breeze.

Puedes encontrar este tutorial en Español aquí: Cómo Probar el Sensor TPS (GM 3.1L, 3.4L) (en:

Symptoms Of A BAD GM TPS

The most common is the CHECK ENGINE LIGHT lit up on the instrument cluster, reminding you that you are not imagining there's something wrong. Here are a couple of others:

  1. TPS diagnostic trouble codes (DTCs) stored in the vehicle's computer's memory.
    1. P0120, P0121, P0122, P0123, P0125, P1121, P1122
  2. Really BAD gas mileage.
  3. Transmission does not shift out of second gear.
  4. No power as you accelerate the vehicle.
  5. Hesitation when you step on the accelerator pedal.
  6. Car does not idle right (unstable/rough idle).

What Tools Do I Need For The TPS Test?

You'll need a multimeter to follow the test steps in this article (don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing). Also, you'll need someone to help you. This assistant will come in handy when doing TEST 3 of this article.

Circuit Descriptions Of The GM 3.1L & 3.4L TPS

How To Test The Throttle Position Sensor (GM 3.1L, 3.4L)

The throttle position sensor on your 3.1L (or 3.4L) GM vehicle has three wires connected to it. Each wire has a specific task to accomplish and with the photos and circuit descriptions below, you'll test each one with your multimeter.

All three circuits start or end at the PCM (that is whether you see the glass as half empty or half full)... so you got to be very careful not to short any of these wires (circuits) to power (12 Volts). If this happens, you'll have a fried Computer on your hands.

  1. Circuit labeled A:
    1. 5 Volts from PCM.
  2. Circuit labeled B:
    1. Sensor Return (Ground) Circuit.
  3. Circuit labeled C:
    1. Throttle Position (TP) Signal Circuit.

An important testing tip I'm going to share with you, that may save you a big headache, is that you should not probe the front of the TP sensor's connector's terminals to test for the 5 Volts and ground signals. It's best to use a tool to pierce the wire... such as a Wire-Piercing Probe (click here so that you can see what one looks like: Wire Piercing Probe) to get to and verify the presence of the signals.

How Does The GM 3.1L and 3.4L TPS Work?

In a nutshell, the throttle position sensor tells the computer how much throttle movement you are causing as you accelerate or decelerate going down the road. Now to be a little more specific... when you crank, start, and get your car moving:

  1. The PCM supplies the TP sensor with 5 Volts and ground.
  2. Once the engines starts and is running (idle), the TPS reports a DC Voltage signal of about .5V to .9V to the PCM. This is the base voltage that the computer needs to have.
  3. Once you throw the car in drive and accelerate the car to get it moving, the throttle position sensor measures whatever movement of the throttle, is caused by your foot action on the accelerator pedal, into a rising DC Voltage signal that the PCM uses to do its little song and dance (calculate fuel injection, ignition timing, transmission shift points, etc).
  4. As you release foot pressure on the accelerator pedal to slow down, the throttle closes and of course the TP Sensor sends the info to the PCM.
  5. This happens continually the whole time you're driving.

Easy stuff, no? Well, the tests to diagnose 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 party started... go to TEST 1.