How To Test The TPS With A Multimeter (1996-2005 3.8L V6 Buick LeSabre)

Eventually, the throttle position sensor (TPS) is gonna wear out and fail. When that happens, the check engine light will usually come on, and you may find one or more of the following trouble codes stored in the PCM's memory: P0121, P0122, or P0123.

The cool thing is that testing the TP sensor is pretty simple. In this tutorial, I'll show you how to check it with a multimeter, and you won't need a scan tool to get the job done.

We're gonna test the TP signal voltage and verify that the sensor is getting power and Ground. I'll explain each step in a simple, step-by-step manner so you can quickly find out if the TP sensor is bad or if the problem lies somewhere else in the circuit.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L V6 Buick LeSabre: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

Symptoms Of A Bad Throttle Position Sensor

The throttle position sensor (TPS) keeps track of how far the throttle plate opens and closes as you press and release the accelerator pedal. It sends this information to the PCM, which uses it to calculate fuel delivery, ignition timing, transmission operation, and several other engine management functions.

When the TP sensor starts to fail or its signal becomes erratic, the PCM no longer knows the throttle plate's exact position. As a result, you're gonna notice one or more driveability problems, and the check engine light will usually come on.

In most cases, you'll find one or more of the following OBD II diagnostic trouble codes stored in the PCM's memory:

• P0121: Throttle Position (TP) Sensor 1 Performance.
• P0122: Throttle Position (TP) Sensor 1 Circuit Low Voltage.
• P0123: Throttle Position (TP) Sensor 1 Circuit High Voltage.
• P1121: TP Sensor Inconsistent With MAP Sensor.

Depending on how the sensor has failed, you may also notice one or more of the following symptoms:

• Engine hesitation: The engine hesitates or stumbles when you press the accelerator pedal.
• Poor acceleration: The engine lacks power when accelerating.
• Reduced fuel economy: Gas mileage drops noticeably.
• High or low idle speed: The engine idles either too high or too low.
• Rough idle: The engine runs rough while idling.
• Starts then stalls: The engine starts but immediately stalls.
• Engine no-start: The engine cranks normally but won't start.

Throttle Position Sensor (TPS) Circuit Descriptions

Throttle Position Sensor (TPS) Circuit Descriptions

As you've probably noticed, the throttle position sensor is connected with a 3-wire electrical connector.

Each one of those three wires has a specific job. Take a look at the table below for a brief description of what each circuit does:

Terminal	Wire	Description
A	Black (BLK)	Sensor Ground
B	Dark Blue (DK BLU)	TPS Signal
C	Gray (GRY)	5 Volts (1994-2000)
	Gray with black stripe (GRY/BLK)	5 Volts (2001-2005)

Where To Buy The TPS And Save

The following links let you comparison shop for a throttle position sensor from two trusted automotive brands. These are quality, direct-fit replacement sensors designed for the 1996-2005 3.8L V6 Buick LeSabre —not cheap knockoff parts that may only last a few weeks.

TEST 1: Testing The TPS Voltage Signal

Our first test is gonna check whether the throttle position sensor is producing a smooth voltage signal that increases as the throttle plate opens and decreases as it closes.

Here's what you should see from a properly working TP sensor:

• Throttle plate closed: The TP signal should be around 0.4 to 0.9 Volts DC.
• Wide-open throttle (WOT): The TP signal should increase smoothly to about 4.5 to 5.0 Volts DC.

When the TP sensor fails, it'll usually behave in one of two ways:

• The voltage stays fixed: The signal remains at one voltage no matter how far the throttle plate is opened.
• The voltage glitches: The signal suddenly drops out, jumps, or skips as you slowly open and close the throttle plate.

To check the TP signal, we're gonna measure the voltage on the dark blue (DK BLU) wire at the TP sensor's 3-wire connector.

The DK BLU wire connects to the female terminal identified by the letter B in the photo above.

IMPORTANT: The TP sensor must remain connected to its electrical connector during this test. The easiest way to access the signal wire is with a wire-piercing probe or a back-probe. You can see what these tools look like and where to buy them here: link goes here.

Let's get started:

1. 1

   Set your multimeter to Volts DC mode.

2. 2

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

3. 3

   Turn the ignition key to the ON position, but don't crank or start the engine.

4. 4

   Probe the dark blue (DK BLU) wire with the red multimeter lead.
   
   The DK BLU wire connects to terminal B of the TP sensor connector.
   
   NOTE: The TP sensor must remain plugged into its connector while you're testing the signal voltage.

5. 5

   Manually open the throttle plate.
   
   You'll get more accurate results by opening and closing the throttle plate directly at the throttle body instead of pressing the accelerator pedal.

6. 6

   With the throttle plate fully closed, your multimeter should register approximately 0.4 to 0.9 Volts DC.

7. 7

   Slowly open the throttle plate.
   
   The voltage should increase smoothly without any sudden jumps or dropouts.
   
   At wide-open throttle, you should see about 4.5 to 5.0 Volts DC.

8. 8

   Slowly close the throttle plate while watching the multimeter.
   
   The voltage should decrease smoothly and return to approximately the same value you observed in STEP 6.

9. 9

   Lightly tap the TP sensor with the plastic handle of a screwdriver while slowly opening and closing the throttle plate.
   
   A good TP sensor will continue producing a smooth, uninterrupted voltage signal. If the voltage suddenly drops out, jumps, or skips while tapping the sensor, the sensor is defective.

Now let's interpret your test results:

CASE 1: The TP sensor voltage increased and decreased smoothly as the throttle plate was opened and closed. This is the correct and expected test result. It confirms that the throttle position sensor is working properly.

It also tells you that the sensor is receiving both its 5-Volt reference signal and Ground from the PCM.

CASE 2: The TP sensor voltage stayed fixed or glitched as the throttle plate was opened and closed. This usually indicates a bad TP sensor. Before replacing it, though, we've got to make sure it's receiving both power and Ground.

Our next step is to verify that the GRY (or GRY/BLK) wire is supplying the sensor with its 5-Volt reference signal. Head over to: TEST 2: Making Sure The TPS Is Receiving 5 Volts.

CASE 3: The multimeter didn't register a TP sensor voltage signal. This usually points to a problem with the TP sensor or one of its circuits.

Before condemning the sensor, we've got to verify that the GRY (or GRY/BLK) wire is supplying the sensor with 5 Volts from the PCM. For that test, head over to: TEST 2: Making Sure The TPS Is Receiving 5 Volts.

TEST 2: Making Sure The TPS Is Receiving 5 Volts

As I mentioned earlier, the throttle position sensor needs two things to work: a 5-Volt reference signal and Ground.

Our next test is gonna verify that the PCM is supplying the TPS with its 5-Volt reference signal. This voltage is carried on the gray (GRY) wire (1994-2000) or the gray with black stripe (GRY/BLK) wire (2001-2005).

This 5-Volt reference wire connects to the female terminal identified by the letter C in the illustration above.

The thing to keep in mind is that the PCM only supplies this reference voltage when the ignition switch is in the ON or START position. For this test, we'll leave the engine OFF and turn the ignition key to the ON position.

Let's get started:

1. 1

   Unplug the throttle position sensor from its electrical connector.

2. 2

   Set your multimeter to Volts DC mode.

3. 3

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

4. 4

   Turn the ignition key to the ON position, but don't crank or start the engine.

5. 5

   Probe terminal C with the red multimeter lead.
   
   Terminal C is the female terminal connected to the GRY or GRY/BLK wire.

6. 6

   Your multimeter should register approximately 5 Volts DC (about 4.5 to 5.0 Volts).

Now let's interpret your test results:

CASE 1: The GRY wire is supplying 5 Volts. This is the correct and expected test result. It confirms that the PCM is supplying the throttle position sensor with its 5-Volt reference signal.

Now that we've verified the reference voltage, our next step is to make sure the TPS has a good Ground. Head over to: TEST 3: Making Sure That The TPS Is Receiving Ground.

CASE 2: The GRY wire isn't supplying 5 Volts. Without this 5-Volt reference signal, the throttle position sensor can't generate a throttle position signal.

In most cases, this is caused by an open in the GRY (GRY/BLK) wire between the TPS connector and the PCM connector, although a PCM problem is also possible (although much less common).

Your next step is to locate and repair the cause of the missing 5-Volt reference signal. Once that problem is fixed, the TPS will be able to operate normally.

TEST 3: Making Sure That The TPS Is Receiving Ground

At this point, our test results have already told us two important things about the throttle position sensor:

• TEST 1: The TP sensor voltage signal didn't respond correctly as the throttle plate was opened and closed.
• TEST 2: The PCM is supplying the sensor with its 5-Volt reference signal.

We've got one last circuit to check. We need to make sure the black (BLK) wire, which connects to terminal A, is providing the TP sensor with a good Ground.

Just like the 5-Volt reference signal, the PCM only supplies this Ground when the ignition switch is in the ON or START position. So, for this test, turn the key to the ON position but leave the engine OFF.

IMPORTANT: Don't apply battery voltage directly to the Ground circuit, or you risk damaging the PCM. The multimeter test outlined below is the safe and accurate way to verify that the Ground circuit is working properly.

Let's get started:

1. 1

   Disconnect the throttle position sensor from its electrical connector.

2. 2

   Set your multimeter to Volts DC mode.

3. 3

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

4. 4

   Turn the ignition key to the ON position, but don't crank or start the engine.

5. 5

   Probe terminal A with the black multimeter lead.
   
   Terminal A is connected to the BLK wire.

6. 6

   Your multimeter should register about 10 to 12 Volts DC.

Now let's interpret your test results:

CASE 1: The BLK wire is providing a good Ground. This is the correct and expected test result.

You can confidently conclude that the throttle position sensor is bad if you've confirmed all of the following:

• TEST 1: The TP sensor voltage signal didn't increase and decrease smoothly as the throttle plate was opened and closed.
• TEST 2: The sensor is receiving its 5-Volt reference signal.
• This test: The sensor is receiving a good Ground.

If you're ready to replace the throttle position sensor, here are the two that I recommend:

• links go here.

CASE 2: The BLK wire isn't providing Ground. Without a good Ground, the throttle position sensor can't generate a throttle position signal.

In most cases, the missing Ground is caused by an open in the BLK wire between the TPS connector and the PCM connector, although a PCM problem is also possible (but much less common).

Our next step is to locate and repair the cause of the missing Ground. Once the Ground circuit is restored, the throttle position sensor will be able to operate normally.

More 3.8L Buick LeSabre Diagnostic Tutorials

You can find a complete list of diagnostic tutorials for the 3.8L V6 Buick LeSabre in this index:

• 3.8L V6 Buick LeSabre Index of Articles.

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

• How To Test For A Blown Head Gasket (1994-2005 3.8L V6 Buick LeSabre).
• Common Causes Of Spark Plug Failure (1994-2005 3.8L V6 Buick LeSabre).
• How To Test Engine Compression (1994-2005 3.8L V6 Buick LeSabre).
• How To Test The CKP Sensor With A Multimeter (1994-2005 3.8L V6 Buick LeSabre).