P0121 -What Does It Mean? (1996-2002 2.0L Mazda 626)

A trouble code P0121: Throttle Position Sensor Performance Problem points to a problem with the throttle position sensor (TPS).

In this article I'll explain what it means and what's involved in diagnosing and repairing its cause.

I'm also going to direct you to the TPS test tutorial that you'll need to test the throttle position sensor.

You can find this tutorial in Spanish here: Código P0121 ¿Qué Significa? (1996-2002 2.0L Mazda 626) (at: autotecnico-online.com).

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

• 2.0L Mazda 626: 1996, 1997, 1998, 1999, 2000, 2001, 2002.

RELATED TROUBLE CODES:

• P0122 -What Does It Mean? (1996-2002 2.0L Mazda 626).
• P0123 -What Does It Mean? (1996-2002 2.0L Mazda 626).

What Does Trouble Code P0121 Mean?

The throttle position sensor (TPS) is tasked with measuring the throttle plate angle as it opens/closes when you step on/off the accelerator pedal.

As the throttle plate opens, the TPS creates a voltage signal that increases.

Once the throttle plate starts to close, the TPS signal voltage decreases.

When the TPS signal voltage does not match the value(s) the fuel injection computer predicts it should be, it sets a trouble code P0121: Throttle Position Sensor Performance Problem.

For a more detailed explanation of how the TPS works, take a look at the section: What Does The Throttle Position Sensor Do?

Common Symptoms Of A P0121 Trouble Code

The throttle position sensor is one of the key input sensors the fuel injection computer uses to calculate how much fuel to inject into the engine.

When the TPS fails, your Mazda 626's engine performance will suffer. You'll see one or more of the following symptoms:

• Check engine light illuminated with a TPS trouble code.
• Rough or low idle.
• Very high idle.
• Engine may start and stall.
• Little to no acceleration

What Does The Throttle Position Sensor Do?

The throttle position sensor's job is to report the changes in the throttle plate's angle as it opens/closes.

And as you're probably aware, the accelerator pedal is connected to the throttle plate via an accelerator cable. So when you step on/off the accelerator pedal, the throttle plate opens/closes.

As you step on the accelerator pedal, the throttle plate opens and more air rushes into the engine. With more air entering the engine, the fuel injection computer needs to inject more fuel.

As you step off the accelerator pedal, less air enters the engine. With less air entering the engine, the fuel injection computer needs to inject less fuel.

When the throttle plate is closed (for example, you've got your foot off the accelerator pedal), the 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?

A bad throttle position sensor is generally the most common cause of a P0121 trouble code.

Unfortunately, a P0121 trouble code can be set by a number of other problems. Here's a basic list of a few other things that can cause it:

• An open-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
• An short-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
• The throttle position sensor is not receiving 5 Volts.
• A bad TPS connector.
• Bad fuel injection computer (although very rare).

How To Diagnose And Repair A P0121 Trouble Code

A trouble code P0121 can be easily troubleshooted and repaired by testing the throttle position sensor.

The TPS can easily be tested with a multimeter to make sure that:

• The TPS signal voltage increases/decreases as the throttle plate opens/closes (and is not stuck producing a low voltage value).
• The TPS is getting 5 Volts from the fuel injection computer.
• The TPS is getting Ground from the fuel injection computer.

You can correctly conclude that the TPS is bad and the cause of the P0121 trouble code if:

• Your test results confirm that the TPS signal voltage DOES NOT increase as you open/close the throttle plate.
• Your test results confirm that the TPS is getting 5 Volts.
• Your test results confirm that the TPS is getting Ground.

I've written a tutorial that'll help you test the TPS on your 1996-2002 2.0L Mazda 626. You can find it here: How To Test The TPS (1994-2002 2.0L Mazda 626).

Where To Buy The TPS And Save

The following links will help you comparison shop for the throttle position sensor of a known and solid automotive name brand (Walker Products):

More 2.0L Mazda 626 Tutorials

You can find a complete list of 2.0L Mazda 626 tutorials in this index:

• Mazda 2.0L Index Of Articles.

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

• How To Test For A Blown Head Gasket (2.0L Mazda 626).
• How To Test The MAF Sensor (1996-1997 2.0L Mazda 626).
• How To Test Engine Compression (2.0L Mazda 626).
• How To Test The Fuel Pump (1994-1999 2.0L Mazda 626).