How To Test TPS Codes: P0121, P0122, P0123 (1997-2001 Jeep 4.0L)

TPS TEST 2: Verifying The TPS Has Power

Terminal number 3, of the TP sensor's harness connector, is the one that feeds the sensor with 5 Volts DC.

If these 5 Volts are missing then the throttle position sensor won't create a throttle angle signal.

So, in this test we'll do a simple voltage test with the multimeter to check for them.

NOTE: You can test for these 5 Volts DC with the TP sensor connected or disconnected to the TPS. I personally prefer to do this test with the TP sensor's connector unplugged.

This is what you'll need to do:

1. 1

   Place your multimeter's dial in Volts DC mode.

2. 2

   Disconnect the TPS from its electrical connector.

3. 3

   Gently probe the terminal labeled with the number 3 with the red multimeter test lead and an appropriate tool.
   
   IMPORTANT: If you probe the front of the TPS harness connector, be careful and don't damage the terminal. Damaging the terminal will require that you replace the connector.

4. 4

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

5. 5

   Have a helper turn the key to the ON position but don't crank or start the engine.

6. 6

   Your multimeter should display 4.5 to 5 Volts.

Let's interpret your test results:

CASE 1: The multimeter registered 4.5 to 5 Volts. This tells you that your 4.0L Jeep's TPS is getting power.

The next step (and the last test) is to check that terminal number 1 of the TP sensor's harness connector is providing Ground. For this test, go to: TPS TEST 3: Verifying The TPS Has Ground.

CASE 2: Multimeter DID NOT register 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 you've just eliminated the TP sensor itself, on your Jeep, as bad.

The two most likely reasons for this are: 1) an open in the wire between the TP sensor's harness connector and the PCM's harness connector or 2) the PCM may be fried (although a bad PCM is very rare).

Although it's beyond the scope of this article to test these two conditions, you have now eliminated the throttle position sensor (TPS) on your Jeep 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: Verifying The TPS Has Ground

Ground is fed to your 4.0L Jeep Grand Cherokee's (Cherokee, Wrangler) TP sensor by terminal number 1 of the sensor's harness connector.

So, in this last test, we'll check this Ground by doing a simple voltage test with the multimeter.

IMPORTANT: Be careful and don't accidentally or intentionally apply power (12 Volts) to this circuit or you'll fry the PCM since it's the PCM that provides this Ground internally.

OK, here are the test steps:

1. 1

   Place your multimeter's dial in Volts DC mode.

2. 2

   Disconnect the TPS from its electrical connector.

3. 3

   Gently probe the terminal labeled with the number 1 with the black multimeter test lead and an appropriate tool.
   
   IMPORTANT: If you probe the front of the TPS harness connector, be careful and don't damage the terminal. Damaging the terminal will require that you replace the connector.

4. 4

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

5. 5

   Have a helper turn the key to the ON position but don't crank or start the engine.

6. 6

   Your multimeter should display 10 to 12 Volts.

Let's take a look at your test results:

CASE 1: The multimeter showed 10 to 12 Volts. This confirms that the PCM and the wire/circuit (that supply this Ground) are OK.

You can conclude that the throttle position sensor is bad if you have confirmed that:

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

CASE 2: The 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 multimeter still doesn't show the indicated voltage, then this indicates a problem with either your Jeep's PCM (internal fault/problem) or an open in the wire between the TPS harness connector and the PCM's harness connector

Although testing these two conditions are beyond the scope of this article, you have now eliminated the throttle position sensor (TPS) on your Jeep as being the cause of the problem and/or the TPS Diagnostic Trouble Code (DTC) lighting up the check engine light (CEL).

More Jeep 4.0L Test Tutorials

If this tutorial was helpful/informative, you can find a complete list of tutorials here: Jeep 4.0L Index Of Articles.

Here's a sample of the tutorials you'll find there:

• How To Test Trouble Code P0135 (1996-2000 Jeep 4.0L).
• How To Test The MAP Sensor (1997-2003 Jeep 4.0L).
• How To Do A Cylinder Balance Test (Jeep 4.0L).
• Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition.
• How To Test The Crankshaft Position Sensor (1994-1996 4.0L Jeep).