P0123 -What Does It Mean? (1996, 1997, 1998, 1999, 2000 1.8L Mazda Protege)

In this tutorial I'll explain what a trouble code P0123: Throttle Position Sensor Circuit High Voltage 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.

En Español You can find this tutorial in Spanish here: P0123 ¿Qué Significa? (1996-2000 1.8L Mazda Protege) (at: autotecnico-online.com).

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

  1. 1.8L Mazda Protege: 1996, 1997, 1998, 1999, 2000.

RELATED TROUBLE CODES:

  1. P0123 -What Does It Mean? (1996-2000 1.8L Mazda Protege).

What Does Trouble Code P0123 Mean?

The throttle position sensor, on your 1.8L Mazda Protege, produces a voltage signal that corresponds to the throttle plate angle.

Specifically, as the throttle plate opens and admits more air into the engine, the TPS signal voltage increases.

As the throttle plate starts to close and less air enters the engine, the TPS signal voltage decreases.

When a failure occurs with the throttle position sensor (or its TPS signal circuit wire) and the TPS signal voltage stays stuck at a high voltage value:

  1. A trouble code P0123: Throttle Position Sensor Circuit High Voltage is set by the fuel injection computer.
  2. The check engine light is illuminated by the fuel injection computer.

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

The throttle position sensor is a key component of your 1.8L Mazda Protege's engine management system.

The fuel injection computer uses the throttle position sensor, along with other input sensors, to calculate the amount of fuel to inject into the engine.

When it fails, your Mazda Protege's engine performance will suffer and 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?

The accelerator pedal is connected to the throttle plate, on the throttle body, by an accelerator cable.

So, when you step on the accelerator pedal:

  1. The throttle plate opens.
  2. More air enters the engine.
  3. The fuel injection computer injects more fuel into the engine.
  4. The engine accelerates.

When you step off the accelerator pedal:

  1. The throttle plate closes.
  2. Less air enters the engine.
  3. The fuel injection computer injects less fuel into the engine.
  4. The engine decelerates.

As mentioned previously, the throttle position sensor is the component responsible for monitoring and reporting the throttle plate angle to the fuel injection computer.

Here are the TPS signal voltages you should see:

  1. At closed throttle (for example, you've got your foot off the accelerator pedal):
    1. The TPS signal voltage is about 0.4 to 0.9 Volts DC.
  1. At wide open throttle (WOT):
    1. The TPS signal voltage is about 4.5 Volts DC.

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?

The most common cause of a P0123 trouble code is a bad throttle position sensor.

Besides a bad TPS, the following problems can also cause a P0123 trouble code:

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

How To Diagnose And Repair A P0123 Trouble Code

A trouble code P0123 can easily be diagnosed and repaired by testing the throttle position sensor.

The TPS test involves:

  1. Making sure that the TPS signal voltage increases/decreases as the throttle plate opens/closes (and is not stuck producing a low voltage value).
  2. Making sure that the TPS is getting 5 Volts from the fuel injection computer.
  3. Making sure that 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 link will help you comparison shop for the throttle position sensor of a known and solid name brand (Walker Products):

More 1.8L Mazda Protege Tutorials

You can find a complete list of 1.8L Mazda Protege tutorials in this index:

  1. Mazda 1.8L Index Of Articles.

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

  1. How To Test Engine Compression (1995-2000 1.8L Mazda Protegé).
  2. How To Test For a Blown Head Gasket (1995-2000 1.8L Mazda Protegé).
  3. How To Test The Fuel Injectors (1999-2000 1.8L Mazda Protegé).
  4. How To Test The MAF Sensor (1999-2000 1.8L Mazda Protegé).
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