How To Test: P0123 OBD II Trouble Code (Honda 2.2L, 2.3L)

OBD II trouble code P0123 TP Sensor Circuit High Voltage is telling you that your Honda's fuel injection computer is seeing a continuous high throttle plate angle that just doesn't correspond to the actual throttle plate opening (and/or current engine operating conditions).

Fortunately, the throttle position sensor (TPS) is one of the easiest sensors to test on your Honda and can be done with a simple multimeter. In this tutorial, I'm gonna' show you how in a step-by-step way.

P0123 Basics You Need To Know

The throttle position sensor (TPS) is connected to the throttle plate and is tasked with measuring the angle of the throttle plate as it opens and closes (the amount it's open or closed is referred to as throttle angle).

In a nutshell: The more the throttle plate opens, as you step on the accelerator pedal, the more fuel the fuel injection computer (known as the PCM = Powertrain Control Module) needs to inject. When the throttle closes, as you let your foot off the accelerator pedal, the less fuel the PCM needs to inject.

This makes the throttle plate angle information a critical part of the computer's fuel injection and ignition system strategy.

Now, when the PCM sees the TP sensor producing a high voltage that indicates an open throttle plate even though other sensor inputs indicate otherwise, it sets a code P0123 TP Sensor Circuit High Voltage and lights up the check engine light (CEL) on your Honda's instrument cluster.

How The Throttle Position Sensor (TPS) Works

To better understand how we're gonna' test the throttle position sensor (TPS), in this tutorial, it's good to know two basic things.

The first is what each of the 3 wires in the TPS connector does. Specifically, what type of signal they carry.

The other, and just as important, we need to know the basics of how the throttle position sensor works.

Don't worry, it's nothing too technical and it's all in plain English:

1. The TP sensor is a 3 wire sensor.
   • Wire labeled with the number 1.
     • Feeds Ground to the TP sensor.
     • Ground is provided by the PCM (internally).
   • Wire labeled with the number 2.
     • Feeds the throttle angle voltage signal to the PCM.
     • This voltage signal varies depending on the amount of throttle plate opening.
   • Wire labeled with the number 3.
     • Feeds power to the TP sensor.
     • In the form of 5 Volts DC and is supplied only with Key On Engine Off (KOEO) or Key On Engine Running (KOER).
     • Power comes directly from the PCM.
2. The TP sensor is a potentiometer. Its resistance changes in response to changes in the throttle plate's angle.
   • With throttle closed, a small voltage is create and sent to the PCM.
     • At closed throttle the TP sensor outputs about 0.5 Volts DC.
   • With throttle open to wide open, a bigger voltage is created and sent to the PCM.
     • At wide open throttle the TP sensor outputs about 4.5 Volts DC.

The key to troubleshooting the throttle position sensor (TPS) is that at closed throttle, the TP voltage signal is about 0.5 Volts DC. As the throttle plate starts to open (as you step on the accelerator pedal and accelerate the engine), the TP voltage signal starts to increase. At wide open throttle, the TP sensor will output about 4.5 Volts DC.

With this bit of information, let's move on to the next subheading.

Symptoms Of A P0123 Diagnostic Trouble Code

What throws a 'throws a monkey wrench into the works' is having the TP sensor read and/or send the wrong throttle plate angle info to the PCM.

You may see one or more of the following symptoms when the throttle position sensor (TPS) fails:

1. Check engine light (CEL) shining nice and bright.
2. DTC P0123 is present.
3. Your Honda fails the state mandated emissions test.
4. Bad gas mileage.
5. Hard start and/or extended cranking time (after shut off).
6. Black smoke coming out of the tailpipe.
7. Hesitation when accelerating your Honda.

Let's find out what are the common causes of a P0123 DTC, in the next subheading.

Common Causes Of A P0123 Trouble Code

The 3 most common cause of trouble code P0123 are:

1. A bad throttle position sensor (TPS).
2. A broken TP sensor connector.
3. A problem in the sensor 3 wires. Specifically, a short in one of them.

Although extremely rare for this to happen, a bad PCM can also cause a false P0123 trouble code.

In this tutorial, I'll help you troubleshoot all three of the above. With this basic info under our belts, let's get testing!

Where To Buy Your TP Sensor And Save

The Honda service manual tells you to replace the entire throttle body when the TP sensor fails but you don't have to.

The TP sensor is sold separately and can be installed as a stand-alone part.

Where can you buy the TP sensor? You can buy it at your local auto parts store but it's gonna' cost a whole lot more. I suggest taking a look at the price of the TP sensor in the following link and compare:

Not sure the TP sensor listed fits your particular Honda? Don't worry, they'll make sure it fits your Honda, once you get to the TP sensor site, or they'll find the right one for you.