How To Test The Throttle Position Sensor (Jeep 4.0L). Troubleshooting Codes: P0121, P0122, P0123

In this tutorial I'm gonna' show you how to test the throttle position sensor (TPS) on your 4.0L Jeep Grand Cherokee (Cherokee, Wrangler) without a scan tool.

This is a very accurate test of the TP sensor done with a multimeter... that will tell you if the TP sensor is fried (or not). You'll be able to also diagnose the following TPS diagnostic trouble codes: P0121, P0122, or P0123.

The TPS test, in this tutorial covers most of the 4.0L equipped Jeep SUVs from 1997 to 2002. To see if your 4.0L Jeep is covered by this tutorial, take a look at the box titled: Applies To: on the right column.

En Español You can find this tutorial in Spanish here: Cómo Probar el Sensor TPS (1997-2001 4.0L Grand Cherokee) (at: autotecnico-online.com).

Symptoms Of A Bad Throttle Position Sensor (TPS)

The TP sensor provides the PCM with throttle plate angle info. This, in layman's terms means that it tells the computer how much you're stepping on (or off) the accelerator.

The throttle angle info the throttle position sensor (TPS) provides is key to engine performance. So, when this bad boy fails, your Jeep is gonna' resent it.

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

  1. Check engine light (CEL) shining nice and bright.
  2. Diagnostic trouble codes (DTC) stored in the PCM's memory:
    1. P0121: TP Sensor Voltage Does Not Agree with MAP sensor.
    2. P0122: TP Sensor Voltage Low.
    3. P0123: TP Sensor Voltage High.
  3. Your Jeep Grand Cherokee (Cherokee, Wrangler) fails the state mandated emissions test.
  4. Bad gas mileage.
  5. Hard start and/or extended cranking time (after shut off).
  6. Black smoke coming out of the tailpipe.
  7. Hesitation when accelerating your Jeep down the road.

TPS TEST 1: Testing The TPS Voltage Signal

Testing The TPS Voltage Signal. How To Test The Throttle Position Sensor (Jeep 4.0L). Troubleshooting Codes: P0121, P0122, P0123

This first TP sensor test involves tapping into the sensor's middle wire, with your multimeter, and manually opening and closing the throttle plate to see if the multimeter registers the changes in the voltage output (of the sensor).

If the throttle position sensor is good, your multimeter will show an increasing voltage signal as you open the throttle plate. It will show a decreasing voltage signal as you close the throttle plate (back to its original closed throttle plate position).

If the sensor has failed... you'll see gaps in the output voltage or the voltage will not increase/decrease with the throttle plate movement.

NOTE: To ensure the accuracy of your test, my suggestion is to test the throttle position sensor (TPS) with the engine warmed up.

OK, let's start:

Part 1

  1. 1

    Place your multimeter in Volts DC mode and with the red multimeter test lead probe the wire labeled with the number 2 in the illustration above. This is the circuit that supplies the TP Signal to the PCM.

    If you don't have a multimeter or need to upgrade yours, check out my recommendation: Abe's Multimeter Recommendation (found at: easyautodiagnostics.com).

    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.)

  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 .4 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: Multimeter registered a smooth increase or decrease in voltage with no gaps. This test result tells you that the throttle position sensor (TPS) on your 4.0L equipped Jeep Grand Cherokee (Cherokee, Wrangler) passed with flying colors.

This test result also confirms that the TP sensor is getting both power (5 volts) and ground.

CASE 2: Multimeter DID NOT register a smooth increase or decrease in voltage, and you saw the voltage reading skip or go dead when tapping the TPS. This test result tells you that the TP sensor has a problem and needs to be replaced.

If your test result showed a lack of change in the voltage output of the TP sensor, I suggest making sure that the sensor is getting both power and ground before replacing it. Go to: TPS TEST 2: Verifying TPS Has Power.

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

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