P0123 -What Does It Mean? (1999, 2000, 2001 1.6L Mazda Protege)

In this article I'll explain what a trouble code P0123: Throttle Position Sensor Circuit High Voltage means and what you'll need to do to diagnose and repair its cause.

En Español You can find this tutorial in Spanish here: Código P0123 ¿Qué Significa? (1999-2001 1.6L Mazda Protege) (at: autotecnico-online.com).

APPLIES TO: This tutorial, on the P0123 OBD II trouble code, applies to the following vehicles:

  1. 1.6L Mazda Protege: 1999, 2000, 2001.

RELATED TROUBLE CODES:

  1. P0122 -What Does It Mean? (1996-2001 1.6L Mazda Protege).

What Does Trouble Code P0123 Mean?

It's the throttle position sensor's job to measure the throttle plate angle as it opens/closes when you step on/off the accelerator pedal.

To be a bit more specific:

  1. When you step on the accelerator pedal and the throttle plate opens, the TPS creates a voltage signal that increases.
  2. As you step off the accelerator pedal and the throttle plate starts to close, the TPS signal voltage decreases.

All of these TPS signal voltage changes are reported directly to your Mazda Protege's fuel injection computer.

When the TPS signal voltage stays stuck in a high voltage value, as the throttle plate opens and closes, the fuel injection computer sets a trouble code P0123: Throttle Position Sensor Circuit High Voltage and lights up the check engine light on the instrument cluster.

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 P0123 Trouble Code

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

Naturally, when the throttle position sensor fails, your Mazda Protege's engine performance will suffer. You'll see one or more of the following symptoms:

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

What Does The Throttle Position Sensor Do?

Since the throttle plate is connected to the accelerator pedal, via an accelerator cable, when you step on or off the accelerator pedal, the throttle plate opens or closes.

To be a bit more specific, when you step on the accelerator pedal:

  1. More air enters the engine and it accelerates.
  2. The fuel injection computer now injects more fuel into the engine to match the increase of air entering it.

When you step off the accelerator pedal:

  1. Less air enters the engine and it decelerates.
  2. The fuel injection computer now injects less fuel into the engine to match the decrease of air entering it.

It's the throttle position sensor's job to monitor and report the changes in the throttle plate angle to the fuel injection computer.

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 or off the accelerator pedal (and that the TPS is functioning correctly).

What Causes A P0123 Trouble Code?

A bad throttle position sensor is usually the most common cause of a P0123 trouble code.

But quite a few other problems can cause a P0123 trouble code. Here's a basic list:

  1. An open-circuit problem in the TPS signal wire between the TPS and the fuel injection computer.
  2. A bad TPS connector.
  3. Bad fuel injection computer (although very rare problem).

How To Diagnose And Repair A P0123 Trouble Code

Troubleshooting and repairing a trouble code P0123 simply involves testing the throttle position sensor to see if it's bad (or not).

The test is done with a multimeter and it involves making sure:

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

You can correctly conclude that the TPS is bad and the cause of the P0123 trouble code if:

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

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 1.6L Mazda Protege Tutorials

You can find a complete list of 1.6L Mazda Protege tutorials in this index:

  1. Mazda 1.6L Index Of Articles.

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

  1. How To Test For A Blown Head Gasket (1.6L Mazda Protegé).
  2. How To Test The Fuel Injectors (1999-2001 1.6L Mazda Protegé).
  3. How To Test Engine Compression (1999-2001 1.6L Mazda Protegé).
  4. How To Test The Ignition Coils (1999-2001 1.6L Mazda Protegé).
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