TEST 2: Verifying Throttle Position Sensor Has Power

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

The TP sensor needs power (in the form of 5 Volts DC) to function. So, in this test section we're gonna' make sure that it's getting power.

The brown with white stripe (BRN/WHT) wire labeled with the number 1 in the photo above is the one that feeds power to the TPS.

  1. 1

    Place your multimeter's dial in Volts DC mode and turn the key on but don't start the engine.

    This will power up the TP sensor's connector.

  2. 2

    Connect the red multimeter test lead to the brown with white stripe (BRN/WHT) wire.

    IMPORTANT: It's not a good idea to probe the front of the TPS connector. Probing the metal terminal of the TPS connector could damage it and require that you replace the connector. Use a back probe or a wire piercing probe instead of probing the front of the connector.

  3. 3

    Connect the black multimeter test lead to a good and clean Ground point on the engine or directly on the negative (-) battery terminal.

  4. 4

    When you've set up the test, have a helper turn the Key On Engine Off (KOEO).

  5. 5

    Your multimeter should display 4.5 to 5 Volts on its screen.

Let's interpret your test result:

CASE 1: The TP sensor is being fed with 4.5 to 5 Volts. This means that the throttle position sensor (TPS) is getting power from your 4.2L Ford fuel injection computer.

The next and last test, is to make sure that the throttle position sensor is getting Ground (from the PCM too). For this test, go to: TEST 3: Verifying Throttle Position Sensor Has Ground.

CASE 2: The TP sensor IS NOT being fed with 4.5 to 5 Volts. Double check all of your connections and repeat the test. If your multimeter still doesn't register the 4.5 to 5 Volts DC...

.. then this test result tells you that the TPS itself is not at fault (and thus causing the TPS trouble code). Without power, your Ford's TPS can't create a throttle angle voltage signal. Although beyond the scope of this tutorial, your next step is to diagnose and restore this missing power.

TEST 3: Verifying Throttle Position Sensor Has Ground

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

If you've reached this point, you have confirmed that:

  1. You have a TPS trouble code (P0121, P0122, P0123).
  2. The TPS is not creating the correct throttle plate angle voltage signal (TEST 1).
  3. Th TPS is getting power in the form of 4.5 to 5 Volts DC (TEST 2).

The last test we need to do is to make sure that it's getting Ground.

The grey with red stripe (GRY/RED) wire, labeled with the number 3 in the photo above, is the one that supplies Ground.

IMPORTANT: Ground is provided directly by the fuel injection computer. Be careful and don't intentionally or accidentally short this wire (circuit) to battery power, or you will fry the fuel injection computer.

OK, here are the test steps:

  1. 1

    Set your multimeter to Volts DC mode.

  2. 2

    Connect the black multimeter test lead to the gray with red stripe (GRY/RED) wire.

    It's not a good idea to probe the front of the TPS connector. Probing the metal terminal of the TPS connector could damage it and require that you replace the connector. Use a back probe or a wire piercing probe instead of probing the front of the connector.

  3. 3

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

  4. 4

    Turn the Key to its ON position but don't start the engine. This will power up the fuel injection computer.

  5. 5

    Your multimeter will display 10 to 12 Volts if the TPS is getting Ground.

CASE 1: Multimeter showed 10 to 12 Volts. This is the correct and expected test result and confirms the TPS is getting Ground.

Taking into account the test results of all 3 test, you have confirmed that:

  1. The TP sensor is not providing a varying voltage signal when manually opening the throttle plate (TEST 1).
  2. The TP sensor is being fed 5 Volts DC (TEST 2).
  3. The TP sensor is being fed Ground (TEST 3).

Therefore, you can conclude that the throttle position sensor is BAD and needs to be replaced (and that this will solve the TP sensor code lighting up the check engine light).

CASE 2: Multimeter DID NOT show 10 to 12 Volts. Double check that you're testing the correct TP sensor harness terminal wire and repeat the test.

If your test result still indicates that the TPS is not getting Ground, then we can conclude that one of two things are causing this lack of Ground:

  1. There's an open in the wire between the TP sensor harness connector and the fuel injection computer's harness connector.
  2. The fuel injection computer has an internal problem (although this is extremely rare).

Although testing these two conditions are beyond the scope of this article, you have now eliminated the throttle position sensor (TPS) on your 4.2L equipped Ford as being the cause of the problem and/or the TP sensor diagnostic trouble code (DTC) lighting up the check engine light (CEL).