How To Test The GM 3.8L Throttle Position Sensor (TPS)

Troubleshooting a BAD throttle position sensor can be easily done with only a multimeter. The TPS is one of those sensors that you can test, to see if it's good or bad, without a scan tool and in this article I'm gonna' show you how to do it in three easy tests.

En Español You can find this tutorial in Spanish here: Cómo Probar El Sensor TPS (GM 3.8L) (at:

NOTE: I've written several ‘How to Test’ articles that are 3.8L GM specific, to see all GM 3.8L V6 test articles, go here: GM 3.8L Index of Articles.

Important Tips And Suggestions

TIP 1: The 1988 thru' 1992 TP sensor does not look like the one in the photo, but you can still apply the testing info (in this article to it).

The 1988-1992 TPS is the adjustable type and the TPS in the photo is the non-adjustable type used from 1993 onward.

TIP 2: Since the1988-1992 TP sensors are the adjustable type, these need to be calibrated to a certain output Voltage (when they're replaced with a new one). If this is the case in your vehicle, please read section: How To Calibrate the 1988-1992 TPS.

TPS TEST 1: Testing The TPS Signal

How To Test The GM 3.8L Throttle Position Sensor (TPS)

We're gonna' jump right into the first test, which is to verify, with your trusty multimeter, that the throttle position sensor (TPS) is creating a viable throttle position signal that the fuel injection computer can use.

Before you actually practice the test on your car, I suggest you read the entire article first and especially take a look at the section: How The Throttle Position Sensor Works. Besides, there's a quiz at the end of the article.

One last thing, to properly test the throttle position sensor on your 3.8L GM car, you'll need a helper. OK, this is what you'll need to do:

Part 1

  1. 1

    If your car starts, crank it up and let it warm up. This test will be more effective at diagnosing a BAD TPS if the engine and the TPS are at normal operating temperature

    In some cases, a BAD throttle position sensor (TPS) will make the car Not Start. So, if your car doesn't Start, don't worry about the engine being warm... continue with the test steps.

  2. 2

    Select Volts DC mode on your multimeter and with the throttle position sensor connected to its electrical connector, probe the sensor's Dark Blue wire. This is the wire that connects to the terminal labeled with the letter B (in the illustration above).

  3. 3

    The BLACK multimeter test lead needs to be grounded at the battery negative terminal. When ready, have your helper turn the Key On but the engine off.

  4. 4

    The multimeter should register about 0.5 to 0.8 Volts DC (you'll usually see something between 0.38 and .042 Volts and this is OK too). If it doesn't, don't worry about it just yet, continue with the other steps.

Part 2

  1. 5

    With your multimeter still connected the the Dark Blue wire and the black test lead grounded at the battery negative terminal (or a good clean metal spot on the engine), open the throttle very slowly and by hand, till it opens to its maximum open position.

  2. 6

    If the TPS is good, the voltage reading on your multimeter will increase and stop at about 4.5 to 4.9 Volts DC (once the throttle is fully open).

  3. 7

    Now, while still observing the multimeter's voltage readings, slowly close the throttle. The Voltage should decrease till it reaches the value you observed in test step 4.

Part 3

  1. 8

    Now, this step involves having a helper to lightly tap on the throttle position sensor (TPS) with the handle of a screw-driver, while you rotate the throttle open and then rotate it back to its closed positions.

    I want to emphasize the words ‘lightly tap’, since the TPS sensor is made out of plastic and could break if beaten.

    The purpose of this screw-driver tapping is to see if it (the tapping) messes up the voltage readings your multimeter. If the TPS sensor is good, the tapping SHOULD NOT have any effect on the voltage readings.

  2. 9

    Repeat step 8 several times to make sure of your multimeter test results.

Interpreting The Results

CASE 1: If the multimeter registered a smooth rise and fall in the voltage reading, this tells you that the TPS is good. No further tests are needed.

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: If the multimeter DID NOT register a smooth rise and fall in Voltage, and the voltage reading skipped or went dead when tapping the TPS, then this tells you that the throttle position sensor (TPS) is BAD. Replace the throttle position sensor.

CASE 3: If 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 power and ground to the sensor. Go to TPS TEST 2.