P0123 - OBD-II Trouble Code

Auto Systems and Repair

Throttle/Pedal Position Sensor/Switch "A" Circuit High Input

Our emissions expert has put together the following information about the P0123 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0123

Fault Code Definition

  • Throttle/Pedal Position Sensor/Switch "A" Circuit High Input

What Does Mean?

The Throttle Position Sensor/Switch is located on the Throttle Body of the Intake Manifold and the Pedal Position Sensor/Switch is part of the Accelerator Pedal assembly. These sensors provide precise input from the driver's foot in terms of how MUCH power is needed from the engine and, how URGENTLY power is needed.  As the Throttle Position Sensor is rotated from its base resting position to full acceleration, typically, it sends an increasing voltage signal to the Powertrain Control Module (PCM). This increasing or decreasing voltage signal is used by the PCM to manage the Air Fuel Ratio and Spark Timing of the engine as well as other Emissions System Components.

Code P0123 triggers when the Throttle/Pedal Sensor/Switch "A" Circuit has high output

Symptoms

  • Check Engine Light will illuminate
  • In many cases, no abnormal symptoms may be noticed
  • In some cases, the engine may be hard starting
  • In some cases, the engine may hesitate during acceleration

Common Problems That Trigger the P0123 Code

  • Defective Throttle or Pedal Position Sensor or Switch
  • Dirty or carbon-filled Throttle Body
  • Torn or jammed floor mats
  • Chafed or corroded Throttle/Pedal Position Sensor, wiring, or connections

Common Misdiagnoses

  • Throttle or Pedal Position Sensor is replaced when the real problem is a poor connection or chafed wiring
  • Throttle Position Sensor/Switch is replaced when the real problem is a dirty or carbon-filled Throttle Body
  • Pedal Position Sensor is replaced when the real problem is torn or damaged floor mats

Polluting Gases Expelled

  • HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
  • CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
  • NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

P0123 Diagnostic Theory for Shops and Technicians
When diagnosing a P0123 code, it is important to record the freeze frame information and then to duplicate the code setting conditions with a test drive while paying close attention to engine load, Mass Air Flow grams per second, RPM, and road speed on a data streaming scan tool. As you drive the vehicle, compare these values to the Throttle/Pedal Position Sensor PID or parameter ID.  The Throttle/Pedal Position Sensor voltage values should rise and fall with changes in engine RPM and engine load. 

Check the Throttle/Pedal Position Sensor connector with the key on-engine off. There needs to be a steady 5 volt reference voltage and an excellent ground. Find and use the proper engine performance wiring diagram to discern the proper color and position of these wires in the connector. 

It is vital to do a key on-engine off test of the Throttle/Pedal Position Sensor by testing the Throttle Position/Pedal Position output from its signal wire. Typically, I operate the Throttle or Pedal manually and watch the rise and fall of its voltage or HTz on a lab scope to see if there are any sensor 'flat spots'  or glitches. I test it again and watch the output on a data streaming scan tool to validate the sensor and Electronic Throttle Module(if equipped) connectivity with the PCM.  

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