How To Test The TPS (1995-2000 4.0L 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.

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

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.

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 Ford Explorer or Mercury Mountaineer 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

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: Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (Amazon affiliate link).

NOTE: The TPS must remain connected to its 3-wire connector to access the TPS voltage signal output of the GRY/WHT wire. You'll need to either use a back probe on the connector or a wire piercing probe on the wire. The wire-piercing probe kit I use (which can also double as a back probe) is this one here: Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

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.
   
   I've labeled the GRY/WHT 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 Won't 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.

TPS TEST 2: Making Sure The TPS Is Receiving 5 Volts

The throttle position sensor (TPS) on your 4.0L equipped Ford vehicle needs power to work. This power comes in the form of 5 Volts DC from the fuel injection computer.

The wire that delivers these 5 volts to the TPS sensor is the brown with white stripe (BRN/WHT) wire, and it connects to the wire labeled with the number 1 in the photo above.

This test will help you to confirm if these 5 Volts are present or not.

CAUTION: When testing the front of the female terminal of the connector, be careful not to damage it with the multimeter test lead. If the metal probe of your multimeter test lead is too thick to probe the terminal, use an appropriate tool to connect the test lead to the terminal.

This is what you'll need to do:

1. 1

   Grab your multimeter and select Volts DC mode on it.

2. 2

   Disconnect the connector from the TP sensor.

3. 3

   Probe the front of the female terminal that connects to the BRN/WHT wire with the red multimeter test lead and an appropriate tool.
   
   I've labeled the BRN/WHT with the number 1 in the photo above.
   
   NOTE: Be careful not to damage the female terminal.

4. 4

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

5. 5

   When everything is set up, have a helper rotate the key to its ON position but don't start the engine.

6. 6

   The multimeter should display 4.5 to 5 Volts DC.

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

CASE 1: The multimeter registered 4.5 to 5 Volts. This is the correct test result and it confirms that the fuel injection computer and the wire are supplying the TPS with power.

The next step is to make sure that the throttle position sensor is getting Ground, go to: TPS TEST 3: Making Sure The TPS Is Receiving Ground.

CASE 2: The multimeter DID NOT register 4.5 to 5 Volts. This test result tells you that the TPS is not receiving power. Without these 5 Volts, the TPS will not function.

The two most likely reasons for this are:

• An open-circuit problem in the wire between the TPS connector and the PCM connector.
• The PCM may be fried (very very rare).

Altho' it's beyond the scope of this tutorial to test these two conditions, you have now eliminated the throttle position sensor (TPS) on your Ford 4.0L as being the cause of the problem and/or the TPS diagnostic trouble code (DTC) lighting up the check engine light (CEL).

TPS TEST 3: Making Sure The TPS Is Receiving Ground

So far you've verified that the TPS is not creating a throttle position signal (TPS TEST 1) and that the TPS is getting power (TPS TEST 2).

The second and last step, before condemning the throttle position sensor as bad, is to verify that it's receiving Ground.

The wire that supplies Ground to the TPS is the gray with red stripe (GRY/RED) wire and it's labeled with the number 3 in the photo above.

CAUTION: The fuel injection computer is the one that provides Ground to the TPS. Be careful and don't accidentally or intentionally apply battery power (12 Volts) to this circuit. If you do, you'll fry the PCM. The multimeter voltage test I'm suggesting in the test instructions below is a safe way to check for this PCM controlled Ground in the GRY/RED wire.

OK, here are the test steps:

1. 1

   Grab your multimeter and select Volts DC mode on it.

2. 2

   Disconnect the connector from the TP sensor.

3. 3

   Probe the front of the female terminal that connects to the GRY/RED wire with the black multimeter test lead and an appropriate tool.
   
   I've labeled the GRY/RED wire with the number 3 in the photo above.
   
   NOTE: Be careful not to damage the female terminal.

4. 4

   Connect the red multimeter test lead to the positive (+) battery terminal.

5. 5

   When everything is set up, have a helper rotate the key to its ON position but don't start the engine.

6. 6

   The multimeter should display 10 to 12 Volts DC.

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

CASE 1: The multimeter showed 10 to 12 Volts. This the correct test result and it confirms that the TPS is getting Ground. You can also conclude that the PCM and the wire/circuit (that supply this Ground) are OK.

This multimeter test result also confirms that the TPS sensor is bad and needs replacement only if you have confirmed that:

• The TPS is not producing a signal that increases/decreases as the throttle plate opens/closes (TEST 1).
• The TPS has power in the form of 5 Volts DC (TEST 2).
• The TPS has Ground (TEST 3).

If you need to replace the TPS, here are my recommendations.

• Walker Products 200-1067 Throttle Position Sensor (Amazon affiliate link).
• Walker Products 200-91067 Throttle Position Sensor (With 3-Wire Connector) (Amazon affiliate link).

CASE 2: The multimeter DID NOT show 10 to 12 Volts. This tells you that the TPS is not getting Ground.

This is usually due to a problem with either the PCM (internal fault/problem) or an open-circuit problem in the wire between the TPS and the PCM itself.

Altho' testing these two conditions are beyond the scope of this tutorial, you have now eliminated the throttle position sensor (TPS) on your Ford vehicle as being the cause of the problem and/or the TPS diagnostic trouble code (DTC) lighting up the check engine light (CEL).

TPS Code Won't Go Away

So you've tested the throttle position sensor (TPS) and according to the test results, the TPS is good, but the check engine light (CEL) keeps coming back on even after you erased the diagnostic trouble code (DTC) from the computer's (PCM) memory. Well, here are a couple of suggestions that might inspire your next diagnostic move:

• The throttle plate's idle-stop screw's factory adjustment has been altered so that the engine could be idled up and mask a miss/misfire and/or rough idle. This increases the TP sensor's signal to the PCM. The PCM doesn't like it and lights up the check engine light (CEL).
• The throttle cable is binding and causing the throttle plate to not fully close.
  • This can be verified by simply having someone inside the vehicle pushing the accelerator pedal to the floor and releasing it, with the engine OFF, while you visually check that the throttle plate and cable are not getting stuck somewhere in their travel.
• The TPS is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:
  • I've found that the best way to test these intermittent problems is to road-test the vehicle with the multimeter hooked up to the TP signal wire with a long wire so that I can comfortably observe the signal going up and down as I or someone else drives.
• The TP sensor's connector is bad, usually the locking tab is broken and the connector has worked itself loose, causing an intermittent false connection.

More 4.0L V6 Ford Explorer And Mercury Mountaineer Diagnostic Tutorials

I've written quite a few 'how to' tutorials that'll help you troubleshoot your 4.0L V6 Ford Explorer or Mercury Mountaineer. You can find the complete list in this index:

• 4.0L V6 Ford Explorer And Mercury Mountaineer Index of Articles.

Here's a sample of the Ford 4.0L V6 articles you'll find:

• How To Test The Starter Motor (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Test The Alternator (1991-2000 4.0L V6 Ford Explorer And Aerostar).
• How To Test Engine Compression (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Test The Thermostat (1996-2003 4.0L V6 Ford Explorer And Mercury Mountaineer).