How To Test The TPS (1995-2000 4.0L Ford Explorer, 1996-1997 4.0L Aerostar, 1995-2000 4.0L Ford Ranger, 1995-2000 4.0L Mazda B4000)

Troubleshooting and testing the throttle position sensor or diagnostic trouble codes: P0121, P0122, P0123 on your 4.0L V6 Ford Explorer (Ranger, or Mercury Mountaineer or Mazda B4000 or Aerostar) can easily be done in three tests and without a scan tool. That's right, no scan tool needed for these tests. This tutorial will show how with step by step instructions. You'll be able to diagnose the TPS as bad or not.

In case you're wondering if this tutorial applies to your particular Ford, Mercury or Mazda pickup or SUV, you can take a look at the application list at the bottom of this page.

To see all Ford 4.0L test tutorials, go here: 4.0L Index Of Articles.

You can find this tutorial in Spanish here: Cómo Probar El Sensor TPS (1995-2000 4.0L Ford Explorer) (at: autotecnico-online.com).

NOTE: This tutorial applies to specific 4.0L equipped vehicles. To see if your vehicle is covered by this tutorial, check out the Applies To: box on the right column. If you need to test the older style TPS (1995 and older), go to this tutorial: How To Test The TPS (1991-1995 4.0L Ford Explorer).

Symptoms Of A Bad TPS

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

  1. TPS diagnostic trouble codes (DTCs) stored in the vehicle's computer's memory. These codes are: P0121, P0122, P0123.
  2. 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.
  3. Transmission does not shift out of second gear. Now, this doesn't happen very often, but it happens.
  4. 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 Ford 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 Ford Explorer (Mercury Mountaineer) 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, go to: TEST 1: Testing The TPS Signal.

TPS TEST 1: Testing The TPS Signal

How To Test The TPS (1995-2000 4.0L Ford Explorer, 1996-1997 4.0L Aerostar, 1995-2000 4.0L Ford Ranger, 1995-2000 4.0L Mazda B4000)

To get this show on the road, we'll dive right into verifying, with a multimeter, if the throttle position sensor is able to create a good throttle position signal that the fuel injection computer can use.

In most cases, when the TPS fails, it will stop creating a voltage signal or the voltage signal will not increase/decrease as you open/close the throttle plate.

If you don't have a multimeter and need to buy one, check out my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.

OK, let's start testing:

Part 1

  1. 1

    Select Volts DC mode on your multimeter

  2. 2

    Probe the gray with white stripe wire (coming out of the throttle position sensor's connector) with the red multimeter test lead, using an appropriate tool to pierce the wire. This is the wire labeled with the number 2 in the photo.

  3. 3

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

  4. 4

    Turn the key On but don't start the engine.

  5. 5

    You should see your multimeter register about 0.9 to 1 Volts.

Part 2

  1. 6

    Manually rotate the throttle plate by hand and observe the multimeter.

  2. 7

    The TPS voltage should increase smoothly as you open the throttle plate to its Wide Open Position. At Wide Open Throttle, the reading on your multimeter should read about 4.5 to 4.9 Volts DC

  3. 8

    Now, slowly release the throttle plate to its original closed position.

  4. 9

    The multimeter's voltage reading should decrease in a smooth and linear fashion back to the exact same voltage you noticed in step 5.

Part 3

  1. 10

    OK, now have your helper lightly tap the TP sensor with the butt of a screw-driver's handle (or something similar).

  2. 11

    As he or she taps with the screw-driver, slowly open the throttle plate to its wide-open position and then slowly release the throttle plate back to its closed position.

  3. 12

    Your multimeter should read a smooth increase and then a smooth decrease in voltage without any gaps or skips in the readings.

  4. 13

    Repeat this (tapping the throttle position sensor) several times to make sure of your multimeter test results.

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

CASE 1: The multimeter registered a smooth increase or decrease in voltage as you opened/closed the throttle plate. This is the correct test result and it confirms that the throttle position sensor is OK. No further testing is necessary.

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 DID NOT register a smooth increase or decrease in voltage. This usually tells you that the throttle position sensor (TPS) is bad.

To confirm that the TPS is bad, the next steps are to make sure that the throttle position sensor is getting power and Ground. Go to: TPS TEST 2: Testing The 5 Volt Reference Signal.

CASE 3: The multimeter DID NOT register any voltage. This usually tells you that the throttle position sensor (TPS) is bad.

To confirm that the TPS is bad, the next steps are to make sure that the throttle position sensor is getting power and Ground. Go to: TPS TEST 2: Testing The 5 Volt Reference Signal.

Ford Vehicles:

  • Aerostar 4.0L
    • 1996, 1997
  • Explorer 4.0L (OHV Only)
    • 1995, 1996, 1997, 1998, 1999, 2000,
  • Ranger 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000

Mazda Vehicles:

  • B4000 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000