How To Test The TPS (1995, 1996, 1997, 1998, 1999, 2000 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer)

You can easily and quickly troubleshoot the throttle position sensor or diagnostic trouble codes: P0121, P0122, P0123 on your 4.0L OHV V6 Ford Explorer (Aerostar, Mercury Mountaineer) with three tests and without a scan tool.

That's right, you don't need a scan tool for these tests, and I'll show you how to perform them with step-by-step instructions. With your test results, you'll easily diagnose the TPS as good or bad.

APPLIES TO: This tutorial applies to the following vehicles:

  • 4.0L V6 OHV Ford Aerostar: 1996, 1997.
  • 4.0L V6 OHV Ford Explorer: 1995, 1996, 1997, 1998, 1999, 2000.
  • 4.0L V6 OHV Mercury Mountaineer: 1998, 1999, 2000.

OHV: Over Head Valve.

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

NOTE: If you need to test the older style TPS (1995 and older), see 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:

  • You'll see one of the following TPS diagnostic trouble codes (DTCs) stored in the vehicle's computer's memory:
    • P0121: Throttle Position Circuit Performance Problem.
    • P0122: Throttle Position Circuit Low Input.
    • P0123: Throttle Position Circuit High Input.
    • P1120: Throttle Position Sensor Out Of Range.
    • P1121: Throttle Position Sensor Inconsistent With MAF Sensor.
    • P1124: Throttle Position Sensor Out Of Self-Test Range.
    • P1125: Throttle Position Sensor Intermittent.
  • 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 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: TPS TEST 1: Testing The TPS Signal.

Where To Buy The TPS And Save

The following links will help you to comparison shop for the 1995-2000 4.0L Ford TPS. I think they'll save you a few bucks:

Not sure if the above TPS fits your particular 4.0L Ford Explorer (Aerostar, Mercury Mountaineer)? Don't worry, once you get to the site they'll make sure it fits by asking you the specifics of your particular Ford vehicle. If it doesn't fit, they'll find you the right one.

TPS TEST 1: Testing The TPS Signal

Testing The TPS Signal. How To Test The TPS (1995, 1996, 1997, 1998, 1999, 2000 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer)

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:

  1. 1

    Place your multimeter in Volts DC mode.

  2. 2

    Connect the red multimeter test lead to the gray with white stripe (GRY/WHT) wire of the TP sensor harness connector.

    The GRY/WHT wire connects to the female terminal identified with the number 2 in the photo above.

    NOTE: The TPS must remain connected to its connector to test the TPS voltage signal.

  3. 3

    Connect the black multimeter test lead directly on the battery negative (-) post.

  4. 4

    Turn the key on but don't crank or start the engine.

  5. 5

    Manually rotate the throttle plate.

    You'll get the best results by opening and closing the throttle plate directly on the throttle body instead of stepping on the accelerator pedal.

  6. 6

    The multimeter should show an increasing voltage as you (or your helper) open up the throttle plate.

  7. 7

    The multimeter should show a decreasing voltage as you begin to close the throttle plate.

  8. 8

    Using a screwdriver's handle, gently tap the TP sensor as you open and close the throttle plate and observe the multimeter.

    The purpose (of tapping the TP sensor with the screwdriver's handle) is to see if the TP sensor shows gaps in the voltage signal. Why? Because a good TP sensor will show a continuous increasing or decreasing voltage signal even while getting tapped by the screw-driver's handle.

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: Making Sure The TPS Is Receiving 5 Volts.

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: Making Sure The TPS Is Receiving 5 Volts.

Ford Vehicles:

  • Aerostar 4.0L
    • 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1998, 1999, 2000