P0121 -What Does It Mean? (1995-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds)

A trouble code P0121: Throttle Position Sensor Performance Problem lets you know that there's a malfunction with the throttle position sensor.

In this tutorial I'll explain what this trouble code means. I'll also talk about some of the tests you can perform to find its cause and solution.

En Español You can find this tutorial in Spanish here: Código P0121 ¿Qué Significa? (1996-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds) (at: autotecnico-online.com).

APPLIES TO: This tutorial, on the P0121 OBD II trouble code, applies to the following vehicles:

  1. 3.8L Buick LeSabre: 1996, 1997, 1998, 1999.
  2. 3.8L Buick Park Avenue: 1996, 1997, 1998, 1999.
  3. 3.8L Buick Regal: 1996, 1997, 1998, 1999.
  4. 3.8L Buick Riviera: 1996, 1997, 1998, 1999.

  1. 3.8L Chevrolet Lumina: 1998, 1999.
  2. 3.8L Chevrolet Monte Carlo: 1998, 1999.

  1. 3.8L Oldsmobile 88 (Eighty-Eight): 1996, 1997, 1998, 1999
  2. 3.8L Oldsmobile 98 (Ninety-Eight): 1996
  3. 3.8L Oldsmobile Intrigue: 1998, 1999
  4. 3.8L Oldsmobile LSS: 1996, 1997, 1998, 1999
  5. 3.8L Oldsmobile Regency: 1997, 1998

  1. 3.8L Pontiac Bonneville: 1996, 1997, 1998, 1999
  2. 3.8L Pontiac Grand Prix: 1997, 1998, 1999

What Does Trouble Code P0121 Mean?

A P0121: Throttle Position Sensor Performance Problem tells you that the fuel injection computer is receiving a throttle plate angle voltage value from the TPS that doesn't correspond to the actual throttle plate angle.

To better understand why this is a problem, take a look at the section: What Does The Throttle Position Sensor Do?

Common Symptoms Of A P0121 Trouble Code

The fuel injection computer uses the throttle position sensor for fuel injection calculations.

Since the TPS is a critical component of the engine management system, when it fails, engine performance will suffer. You'll see one or more of the following symptoms:

  1. Rough or low idle.
  2. Very high idle.
  3. Engine may start and stall.
  4. Little to no acceleration

What Does The Throttle Position Sensor Do?

Simply put, the throttle position sensor's job is to inform the fuel injection computer the throttle plate's angle as it opens/closes.

Since the throttle plate is connected to the accelerator pedal, it opens/closes as you step on/off the accelerator pedal.

When the throttle plate is in its closed position (like when the engine is idling and your foot is off the accelerator pedal), the TPS signal voltage is about 0.4 to 0.9 Volts DC.

Now, as the throttle plate opens, the throttle position sensor signal voltage increases. At wide open throttle (WOT), the TPS signal voltage is around 4.5 Volts.

As the throttle plate closes, the throttle position sensor signal voltage decreases.

As long as the fuel injection computer sees the TPS voltage signal increasing/decreasing, it knows you're stepping on/off the accelerator pedal (and that the TPS is functioning correctly).

What Causes A P0121 Trouble Code?

The most common cause of a P0121 trouble code is a bad throttle position sensor.

I've also seen a bad MAP sensor cause this trouble code (since both the MAP and TPS share the same 5 Volt DC power circuit).

Other things that can cause a P0121 trouble code are:

  1. Bad MAP sensor (specifically, a MAP sensor that has an internal short-circuit problem causing the TPS to not receive 5 Volts).
  2. An open-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
  3. A short-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
  4. A bad TPS connector.
  5. TPS not receiving 5 Volts due to an open-circuit or short-circuit problem in its 5 Volt wire.
  6. Bad fuel injection computer (although very rare).

How To Diagnose And Repair A P0121 Trouble Code

Troubleshooting and resolving a P0121 involves testing the throttle position sensor with the MAP sensor disconnected from its electrical connector.

The test involves making sure that the throttle position sensor's signal voltage increases/decreases as you manually open/close the throttle plate.

You'll also need to make sure that the throttle position sensor is getting 5 Volts and Ground from the fuel injection computer.

If the TPS signal voltage DOES NOT increase as you open/close the throttle plate and the sensor is getting 5 Volts and Ground, then you can conclude that the sensor is bad and needs to be replaced.

If the TPS is good, then the next step is to test the MAP sensor.

You can find the TPS test explained in detail here: How To Test The GM 3.8L Throttle Position Sensor (TPS).

You can find the MAP sensor test explained in detail here: How To Test The MAP Sensor (1995-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds).

Where To Buy The TPS And Save

The following links will help you comparison shop for the throttle position sensor:

More 3.8L V6 GM Tutorials

You can find a complete list of 3.8L GM V6 tutorials in this index:

  1. GM 3.8L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

  1. How To Test The MAF Sensor (GM 3.8L V6 1996-2005).
  2. How To Test A Does Not Crank Condition -Case Study (GM 3.8L).
  3. GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
  4. How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L).
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