What Does The Throttle Position Sensor Do? (1991-2003 4.0L Ford Explorer, Aerostar, And Mercury Mountaineer)

The throttle position sensor (TPS) is a critical component of the engine management system on your 4.0L Ford Explorer (Mercury Mountaineer).

Sooner or later, it will fail, and when it does, engine performance will suffer. In this article, I'll answer some of the most commonly asked questions about the TPS on the 1991-2003 4.0L Ford Explorer (Mercury Mountaineer).

APPLIES TO: This tutorial applies to the following vehicles:

• 4.0L V6 Ford Aerostar: 1991, 1992, 1993, 1994, 1995, 1996, 1997.
• 4.0L V6 Ford Explorer: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 4.0L V6 Mercury Mountaineer: 1998, 1999, 2000, 2001, 2002.

What Does The Throttle Position Sensor (TPS) Do?

The throttle position sensor measures the throttle plate angle as you step on and step off the accelerator pedal.

Specifically, the throttle position sensor produces a voltage signal that increases as the throttle plate opens.

When the throttle plate begins to close, the TPS voltage signal decreases.

The TPS then sends this information to your vehicle's fuel injection computer.

The fuel injection computer then uses this information to adjust ignition timing, inject more fuel, and a host of other functions to keep the engine running optimally and efficiently.

What Happens When The Throttle Position Sensor (TPS) Goes Bad?

The throttle position sensor usually fails in one of two ways:

• It malfunctions completely and does not measure nor report the throttle plate angle.
• It malfunctions intermittently. Most of the time, it'll work fine, but every now and then, it won't.

In the next section, you'll find a list of common symptoms you'll see when the TPS fails.

What Problems Can A Throttle Position Sensor (TPS) Cause?

When the throttle position sensor fails, you'll see one or more of the following symptoms:

• Check engine light illuminated in the instrument cluster.
• If OBD II equipped, you'll see one of the following TPS trouble codes registered in the fuel injection computer's memory:
  • Code P0121: Throttle Position Sensor Performance Problem.
  • Code P0122: Throttle Position Sensor Signal Voltage Low.
  • Code P0123: Throttle Position Sensor Signal Voltage High.
• If OBD I equipped, you'll see one of the following TPS trouble codes registered in the fuel injection computer's memory:
  • Code 23: Throttle Position Sensor Out Of Self-Test Range.
  • Code 53: Throttle Position Sensor Circuit Above Maximum Voltage.
  • Code 63: Throttle Position Sensor Circuit Below Minimum Voltage.
  • Code 121: Closed Throttle Position Sensor Voltage Higher Or Lower Than Expected.
  • Code 122: Throttle Position Sensor Circuit Below Minimum Voltage.
  • Code 123: Throttle Position Sensor Circuit Above Maximum Voltage.
  • Code 124: Throttle Position Sensor Voltage Higher Than Expected.
  • Code 125: Throttle Position Sensor Voltage Lower Than Expected.
• Engine hesitates when you step on the accelerator pedal.
• Lack of power when accelerating the vehicle.
• Bad gas mileage.
• Engine idle either too high or too low.
• Rough engine idle.
• The engine may start and immediately stall.
• The engine cranks but does not start.

How Can You Tell If The Throttle Position Sensor (TPS) Is Bad?

The surest way to determine that the TPS has failed is to test it. Thankfully, we can easily and quickly test the TPS with a multimeter.

In the next section, I'll go into some of the specifics of the TPS test.

How Can I Find Out If The Throttle Position Sensor (TPS) Is Bad?

When the throttle position sensor fails, it will stop measuring and reporting the throttle plate angle. In other words, its output voltage signal will stay stuck in one value as the throttle plate opens/closes.

We can easily confirm if this is happening by connecting a multimeter to the TPS output signal wire and checking the voltage output as we open/close the throttle plate.

This is the throttle position sensor's test procedure:

1. Checking for TPS trouble code stored in the fuel injection computer's memory.
2. Making sure that the TPS is getting power.
3. Making sure that the TPS is getting Ground.
4. Making sure that the TPS signal increases/decreases as the throttle plate is opened/closed.

If the TPS has failed, its output voltage signal will not increase/decrease as you open/close the throttle plate.

You can find the throttle position sensor test explained in a step-by-step manner here:

• How To Test The TPS (1997-2001 4.0L SOHC Ford Explorer).
• How To Test The TPS (1995-2000 4.0L Ford Explorer, Aerostar, And Mercury Mountaineer).

Can I Drive My Vehicle With A Bad Throttle Position Sensor (TPS)?

When the throttle position sensor fails, the fuel injection computer activates its 'limp mode' (also known as 'limp home mode' or 'limp-in mode').

The 'limp mode' limits engine performance to a considerable degree to ensure the fault doesn't cause any severe damage to the engine.

Depending on the severity of the symptoms that the 'limp mode' or the faulty throttle position sensor is causing, you won't be able to drive the vehicle safely.

Where Can I Buy The Throttle Position Sensor (TPS)?

The following links will help you to comparison shop for the throttle position sensor (of known professional automotive brands- NO knockoffs) for your 4.0L Ford Explorer (Mercury Mountaineer).

NOTE: Not sure if the above TPS fits your particular 4.0L Ford Explorer (Mercury Mountaineer)? Don't worry. Once you get to the site, they'll ask you for the specifics of your vehicle and check. If it isn't, they'll find you the correct one.

More 4.0L Ford Explorer (Mercury Mountaineer) Tutorials

You can find a complete list of 4.0L Ford Explorer (Mercury Mountaineer) tutorials in this index:

• Ford 4.0L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

• How To Test The Fuel Pump (Ford 4.0L Aerostar, Ranger, Explorer).
• How To Test Engine Compression (1991-2010 4.0L Ford Explorer, Mercury Mountaineer).
• How To Troubleshoot A No Start (Ford 4.0L).
• How To Test For A Blown Head Gasket (Ford 4.0L Explorer, Aerostar And Mercury Mountaineer).