How To Test The TPS With A Multimeter (1997-2005 3.1L V6 Buick Century)

Sooner or later, the throttle position sensor (TPS) is gonna fail. And when it does, you're usually going to see one of the following diagnostic trouble codes lighting up the check engine light: P0121, P0122, or P0123.

In this tutorial, I'm going to show you how to test the TP sensor using only a multimeter. No scan tool is required.

We'll check the TP signal voltage and also make sure the sensor is getting power and Ground. All the test steps are explained in a step-by-step manner so that you easily and quickly find out if the TP sensor is bad.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.1L V6 Buick Century: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004.

Symptoms Of A Bad Throttle Position Sensor

As you probably already know, the throttle position sensor's job is to let the computer (PCM → Powertrain Control Module) know how much the throttle plate is opening as you step on or step off the gas pedal.

Since the PCM depends on this information to properly manage the engine, a bad TPS is going to create noticeable driveability problems you can't ignore.

When that happens, the check engine light will usually come on with one or more of the following OBD II diagnostic trouble codes:

• 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.

You're also going to see one or more of the following symptoms:

• Engine hesitation: The engine hesitates when you step on the accelerator pedal.
• Lack of power: The engine lacks power when accelerating the vehicle.
• Poor fuel economy: Your gas mileage is gonna drop noticeably.
• High or low idle: The engine idles either too high or too low.
• Rough idle: The engine runs rough at idle.
• Starts then stalls: The engine starts but immediately stalls.
• Cranks but won't start: The engine cranks but does not start.

Throttle Position Sensor (TPS) Circuit Descriptions

I'm sure you've already noticed that the throttle position sensor has three wires coming out of its pigtail connector.

Each wire has a specific role, and the table below gives a brief description of each:

Terminal	Wire	Description
A	Grey (GRY)	5 Volts
B	Dark Blue (DK BLU)	TPS Signal
C	Black (BLK)	Ground

Where To Buy The TPS And Save

The following links will let you comparison shop for a throttle position sensor from two well-known professional automotive brands. These are quality, direct-fit replacement sensors —not cheap knockoff parts that may only last a few weeks. Both are designed to fit the 1997-2005 3.1L V6 Buick Century.

TEST 1: Testing The TPS Voltage Signal

Our first test is going to verify that the throttle position sensor is producing a variable voltage signal that increases as you open the throttle plate and decreases as you close it.

Here's what we need to know:

• With the throttle plate completely closed, you should see a TPS signal of about 0.4 to 0.9 Volts DC.
• At wide-open throttle, that signal should increase to around 4.5 to 5.0 Volts DC.

When the TP sensor fails, it'll usually do one of two things:

• It'll either stay stuck at one voltage regardless of throttle plate angle.
• The voltage signal will glitch or skip as you slowly open and close the throttle plate.

To the TPS voltage signal, we're gonna connect a multimeter to the dark blue (DK BLU) wire of the TP sensor's 3-wire connector.

The DK BLU wire connects to the female terminal marked with the letter C in the photo above.

IMPORTANT: The throttle position sensor must remain connected to its connector in order to access the signal in the wire. It would be best to use either a wire piercing probe or a back probe. You can see what this tool looks like and where to buy it here: Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

Let's get started:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

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

3. 3

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

4. 4

   Connect the red multimeter test lead to the dark blue (DK BLU) wire of the TP sensor harness connector.
   
   The DK BLU wire connects to the terminal labeled with the letter C in the photo above.
   
   NOTE: The TPS must remain connected to its connector to test the TPS voltage signal.

5. 5

   Manually rotate the throttle plate to its open position.
   
   You'll get the best results by opening and closing the throttle plate directly on the throttle body instead of stepping on the accelerator pedal.

6. 6

   At this point, your multimeter should register a voltage between 0.4 to 0.9 Volts DC.

7. 7

   Slowly open up the throttle plate.
   
   The multimeter should show an increasing voltage reading.
   
   At wide open throttle (WOT), you should see a value between 4.5 to 5 Volts DC.

8. 8

   Slowly close the throttle plate as you observer the multimeter.
   
   The multimeter should show a decreasing voltage reading.
   
   At the closed throttle position, you should see the same voltage value you observed in STEP 6.

9. 9

   Using a screwdriver's handle, gently tap the TP sensor as you open and close the throttle plate and observe the multimeter.
   
   The purpose (of tapping the TP sensor with the screwdriver's handle) is to see if the TP sensor shows gaps in the voltage signal. Why? Because a good TP sensor will show a continuous increasing or decreasing voltage signal even while getting tapped by the screw-driver's handle.

Let's analyze your test results:

CASE 1: The TP sensor voltage changed smoothly as the throttle plate was opened and closed. This is the expected test result and confirms that the throttle position sensor is operating correctly.

It also tells you that the sensor is receiving both power (5 Volts) and Ground from your Buick Century's PCM.

CASE 2: The TP sensor voltage did not change, or glitched, as the throttle plate was opened and closed. This usually means the TP sensor has a problem, but before we replace it, we'll don a two more tests.

Your next step is to verify that the GRY wire is supplying the sensor with power (5 Volts). Head over to: TEST 2: Making Sure The TPS Is Receiving 5 Volts.

CASE 3: The multimeter did not register any TP sensor voltage. This usually points to a problem with the TP sensor or its circuits.

Before condemning the sensor, we'll make sure the GRY wire is supplying 5 Volts (which come from the PCM). For this test, head over to: TEST 2: Making Sure The TPS Is Receiving 5 Volts.

TEST 2: Making Sure The TPS Is Receiving 5 Volts

For the throttle position sensor on your Buick Century to do its job, it needs two things: power and Ground. The good news is that checking for both is pretty easy.

In this section, we're going to make sure the TPS is getting its 5 Volt reference signal from the PCM. This 5 Volt supply is carried by the gray (GRY) wire, which connects to the female terminal I've labeled A in the illustration above.

The PCM only supplies these 5 Volts when the ignition key is in the ON or START position. For this test, we'll place the key in the ON position but keep the engine OFF.

Let's get testing:

1. 1

   Disconnect the TPS from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

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

4. 4

   Turn the key the ON position but don't start the engine.

5. 5

   With the red multimeter test lead, gently probe the female terminal labeled with the letter A.
   
   Terminal A connects to the GRY wire of the connector.

6. 6

   Your multimeter should read 4.5 to 5 Volts DC.

Let's analyze your test results:

CASE 1: The GRY wire is delivering 5 Volts. This is the correct test result and confirms that the PCM is supplying the TPS with its 5-Volt reference signal.

Now that the 5-Volt reference signal is verified, our next step is to make sure the TPS has a good Ground. Go to: TEST 3: Making Sure That The TPS Is Receiving Ground.

CASE 2: The GRY wire IS NOT delivering 5 Volts. Without this voltage, the throttle position sensor cannot produce a throttle position signal.

In most cases, the missing 5 Volt reference signal is caused by an open-circuit problem in the GRY wire somewhere between the TPS connector and the PCM connector.

Your next step is to locate and repair that problem. Once this 5 Volt power supply is restored to the circuit (wire), the TPS will function again.

TEST 3: Making Sure That The TPS Is Receiving Ground

So far, your multimeter test results have told us two important things about your Buick Century's TPS:

• The TPS voltage signal did not respond correctly as the throttle plate was opened and closed (TEST 1).
• The PCM is supplying the TPS with a 5 Volt power supply (TEST 2).

That leaves us with one last thing to check. We need to make sure the black (BLK) wire connected to the terminal labeled B is providing the TPS with a good Ground.

The PCM only supplies this Ground when the ignition key is in the ON or START position. For this test, we'll place the key in the ON position but keep the engine OFF.

IMPORTANT: Be careful not to accidentally apply battery voltage to the Ground circuit (wire), or you're gonna deep-fry the PCM. The multimeter voltage test I'm outlining below is both a safe and accurate method for checking the Ground circuit.

These are the test steps:

1. 1

   Disconnect the TPS from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

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

4. 4

   Turn the key the ON position but don't start the engine.

5. 5

   With the black multimeter test lead, gently probe the terminal labeled with the letter B.
   
   Terminal B connects to the BLK wire of the connector.

6. 6

   Your multimeter should read 10 to 12 Volts DC.

Let's analyze your test results:

CASE 1: The BLK wire is delivering Ground. This is the correct test result.

You can correctly conclude that the TPS is defective if you have:

• Confirmed that the TPS voltage signal does not increase/decrease as you opened/closed the throttle plate (TEST 1).
• Confirmed that the TPS is getting 5 Volts (TEST 2).
• Confirmed, in this test section, that the TPS is getting Ground.

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

• Dorman 977-030 Throttle Position Sensor (Amazon affiliate link).
• ACDelco Gold 213-4668 (88864359) Throttle Position Sensor (Amazon affiliate link).

CASE 2: The BLK wire IS NOT delivering Ground. Without this PCM-supplied Ground, the throttle position sensor cannot produce a throttle position signal.

In most cases, this missing Ground signal is caused by an open-circuit problem in the BLK wire somewhere between the TPS connector and the PCM connector.

Your next step is to locate and repair that problem. Once this Ground is restored to the circuit (wire), the TPS will function again.

More 3.1L V6 Buick Century Diagnostic Tutorials

You can find a complete list of diagnostic tutorials for the 3.1L V6 Buick Century in this index:

• 3.1L V6 Buick Century Index of Articles.

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

• How To Test Engine Compression (1994-2005 3.1L V6 Buick Century).
• How To Test For A Blown Head Gasket (1994-2005 3.1L V6 Buick Century).
• Common Causes Of Spark Plug Failure (1994-2005 3.1L V6 Buick Century).
• How To Test The 24X Crankshaft Position Sensor (1994-2005 3.1L V6 Buick Century).