How To Test The Throttle Position Sensor (4.2L Ford)

The throttle position sensor (TPS) is probably one of the easiest sensors to test on your 4.2L equipped Ford pick up or van.

In this tutorial I'll show you how to test it with a simple multimeter (no scan tool required).

The contents of this tutorial at a glance:

  1. Symptoms Of A Bad Throttle Position Sensor.
  2. TEST 1: Testing The Throttle Position Sensor Voltage Signal.
  3. TEST 2: Verifying Throttle Position Sensor Has Power.
  4. TEST 3: Verifying Throttle Position Sensor Has Ground.
  5. Where To Buy Your Throttle Position Sensor And Save.
  6. More 4.2L Ford Diagnostic Tutorials.

En Español Puedes encontrar este tutorial en Español aquí: Cómo Probar el Sensor TPS (4.2L Ford F150, E150, E250) (en: autotecnico-online.com).

Symptoms Of A Bad Throttle Position Sensor

The TP sensor is one of several mission-critical sensors the PCM uses to calculate the amount of fuel to inject into the engine, advance/retard ignition timing, etc.

So, a bad throttle position sensor (TPS) will cause some pretty obvious symptoms among them a check engine light (CEL) shining nice and bright on your 4.2L Ford pick-up or van.

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

  1. A TPS diagnostic trouble code (DTC) stored in the PCM's memory:
    1. P0121: Throttle Position (TP) Circuit Performance Problem.
    2. P0122: Throttle Position (TP) Circuit Low Input.
    3. P0123: Throttle Position (TP) Circuit High Input.
  2. Your 4.2L Ford pick up (van) fails the smog check (state mandated emissions test).
  3. Bad gas mileage.
  4. Hard start and/or extended cranking time (after shut off).
  5. Black smoke coming out of the tailpipe.
  6. Hesitation when accelerating your vehicle down the road.

Thankfully, the TPS can be tested without a scan tool and in the next section we'll start with the very first test.

TEST 1: Testing The Throttle Position Sensor Voltage Signal

How To Test The Throttle Position Sensor (4.2L Ford)

The get our TPS diagnostic on the road, the first test we'll do is to tap into the TP signal wire (of the TPS connector) with a multimeter.

Once we've got the multimeter connected, we're gonna' bench test the throttle position sensor (while it's still bolted to the throttle body and connected to its connector).

This is a pretty simple and straight-forward test and in the test steps below it's all explained in detail.

NOTE: The throttle position sensor has to remain connected to its connector for this test to work (this is where a wire piercing probe comes in handy to get to the signal inside the wire. To see what one looks like, click here: Wire Piercing Probe Tool.)

OK, let's start:

  1. 1

    Place your multimeter in Volts DC mode and with the RED multimeter lead probe the middle wire of the sensor's connector. This is the wire that connects to TPS terminal number 2 in the illustration above.

  2. 2

    Ground the BLACK multimeter test lead on the battery negative terminal. Have you helper turn the Key On, but don't start the engine (this will power up the TP sensor).

  3. 3

    Your multimeter should report a voltage between .2 to .9 Volts DC. If your multimeter doesn't, don't worry about it just yet, continue with the other steps.

Part 2

  1. 4

    Now, slowly open the throttle (by hand and from the engine compartment) while you observe the change in voltage numbers on your multimeter.

    For this test result to be accurate, you need to open the throttle by hand and not from inside the vehicle.

  2. 5

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

  3. 6

    Now, slowly close the throttle. As the throttle is closing, you should see the voltage decrease smoothly and without any gaps or skips, to the exact same voltage you noticed in step 4.

Part 3

  1. 7

    OK, now you'll need someone to help you 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 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.

  2. 8

    Repeat step 7 several times to make sure of your multimeter test results.

Let's take a look at your test results:

CASE 1: The throttle angle voltage increased and decreased as you opened and closed the throttle plate. This confirms that your 4.2L Ford's TP sensor is OK and not defective.

CASE 2: The throttle angle voltage DID NOT increase (and/or decrease) as you opened and closed the throttle plate. This tells you that the TPS is bad and causing the TPS trouble code lighting up the check engine light (CEL) on your 4.2L Ford.

Before you run out and buy it... I'm gonna' suggest that you do two more tests. One is to check that the TPS is getting power. The other is to check that it's ground. To check that the TPS is getting power, go to: TEST 2: Verifying Throttle Position Sensor Has Power.

CASE 3: Multimeter DID NOT register any voltage, this test result doesn't condemn the TP sensor as BAD just yet. Why? Because...

... the TP sensor may be missing either power or ground. So the next step is to check that the TP sensor is getting power, go to TEST 2: Verifying Throttle Position Sensor Has Power.