How To Test The TPS (1997, 1998, 1999, 2000, 2001 4.0L SOHC Ford Explorer)

In this tutorial I'm gonna' explain how to test the throttle position sensor (TPS) with a multimeter.

This is a very easy test and what makes it so is that the TPS is very accessible. You'll be able to find out if the TPS is bad or not in 3 simple tests.

NOTE: This tutorial applies to the following vehicles:

  1. 4.0L SOHC Ford Explorer: 1997, 1998, 1999, 2000, 2001.

Symptoms Of A Bad Throttle Position Sensor

The throttle position sensor's job is to report the throttle plate angle as you step on or step off the accelerator pedal.

To be a bit more specific, when you step on the accelerator pedal, the throttle plate opens. When you step off the accelerator pedal, the throttle plate closes.

As the throttle plate opens and closes, the throttle position sensor sends a voltage signal to the fuel injection computer that corresponds to the throttle plate's angle.

Since the throttle position sensor is constantly monitored (when the engine is running), when it fails the fuel injection computer will set a TPS trouble code and it'll light up the check engine light. You'll see one of the following trouble codes:

  1. P0121: Throttle Position Sensor.
  2. P0122: Throttle Position Sensor.
  3. P0123: Throttle Position Sensor.

You're also gonna' see one or more of the following symptoms:

  1. Hesitation when accelerating the engine.
  2. Lack of power.
  3. Bad gas mileage.

TEST 1: Testing The TPS Voltage Signal

How To Test The TPS (1997, 1998, 1999, 2000, 2001 4.0L SOHC Ford Explorer)

For our first test we're gonna' connect our multimeter to the TPS signal wire of the sensor's electrical connector.

The wire that we need to connect the multimeter to, is the gray with white stripe (GRY/WHT) wire. This wire is labeled with the number 2 in the photo above.

What we're looking for is for the TPS signal voltage to increase/decrease as we open/close the throttle plate (in the throttle body).

If the throttle position sensor is bad, it'll usually stay stuck producing a fixed voltage value as you open/close the throttle plate.

NOTE: The throttle position sensor must remain connected to its electrical connector to able to read the TPS signal voltage. You'll need to use a back probe on the connector or a wire piercing probe on the wire. You can see an example of this tool here: Wire Piercing Probe.

Alright, these are the test steps:

  1. 1

    Turn the key to the ON position but don't start the engine.

  2. 2

    Place your multimeter in Volts DC mode.

  3. 3

    Connect the red multimeter test lead to the wire labeled with the number 2.

    NOTE: The TP sensor must remain connected to its electrical connector.

  4. 4

    Connect the black multimeter test lead to the battery negative (-) terminal

  5. 5

    Your multimeter should report a voltage between 0.2 to 0.9 Volts DC with the throttle plate closed. If your multimeter doesn't, don't worry about it just yet, continue with the other steps.

  6. 6

    Slowly open the throttle plate (by hand and from the engine compartment). The voltage numbers should increase as the throttle plate opens.

    This increase in voltage should be smooth and without any gaps or skips. Once the throttle plate is wide open, your multimeter should read somewhere between 3.5 to 4.5 Volts DC.

  7. 7

    Slowly close the throttle plate. As the throttle plate is closing, you should see the voltage decrease smoothly and without any gaps or skips, to the exact same voltage you noticed in step 3.

  8. 8

    Lightly tap on the throttle position sensor with the handle of a screw-driver (or something similar, and I want to emphasize the words ‘lightly tap’) as you slowly open and close the throttle plate and observe the multimeter.

    If the TPS is bad, the tapping will cause the voltage numbers to skip or go blank. If the TPS is OK, the tapping will have no effect on the voltage numbers.

  9. 9

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

Let's take a look at your test results:

CASE 1: The TPS signal voltage increased/decreased as you opened/closed the throttle plate. This is the correct test result. It tells you that the throttle position sensor IS NOT defective.

This test result also confirms that the TPS is getting both power and Ground from the fuel injection computer.

CASE 2: The TPS signal voltage DID NOT increase/decrease as you opened/closed the throttle plate. This test result usually tells you that the TPS is bad.

To make sure that the throttle position sensor is bad, you need to make sure that it's receiving power and Ground. Go to: TEST 2: Making Sure The TPS Is Getting 5 Volts.

CASE 3: The multimeter DID NOT register any voltage. This test result usually indicates that the TPS is not getting either 5 Volts or Ground.

We can easily find out if power or Ground are not reaching the throttle position sensor. Go to: TEST 2: Making Sure The TPS Is Getting 5 Volts.