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What does code P0120 mean?

OBD-II Code P0120 is defined as a Throttle/Pedal Position Sensor/Switch "A" Circuit Malfunction

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.

The Pedal Position Sensor is used on vehicles equipped with an Electronic Throttle Body or "Drive by Wire" system. In these systems, the Pedal Position Sensor translates the movement of the driver's foot into a falling or rising voltage that is sent to the PCM. This changing voltage signal is used by the PCM to control the opening or closing of the Electronic Throttle Body, the Air Fuel Ratio, the Ignition Spark Timing, and other Emissions System Components. In a "Drive by Wire" system, the Throttle Position Sensor (the one mounted on the Throttle Body) is used to send a feedback voltage that verifies whether or not the desired Throttle Body opening has been attained.

When the throttle/pedal sensor/switch "A" circuit malfunctions, code P0120 is triggered.

Related OBD-II Codes

  • P0124 - Throttle/Pedal Position Sensor/Switch "A" Circuit Intermittent

P0120 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 P0120 Code

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

Common Misdiagnoses

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

P0120 Diagnostic Theory for Shops and Technicians

The Throttle Position Sensor or Pedal Position Sensor is a rotational potentiometer that typically receives a 5-volt reference voltage from the PCM and then outputs a change to that voltage depending on the position of the potentiometer. When the Throttle or pedal is in the resting or engine idle position, the voltage output from either of these sensors is low, usually in the .5- to 1-volt range. As the Throttle is opened and/or the pedal is depressed, the voltage rises to around 2.5 volts at half throttle and to 4.5 to 5 volts at Wide Open Throttle.

Whenever P0120–P0124 codes are triggered and set in the PCM's memory, the first thing to do is to verify the code(s). A very effective way to verify the code is to inspect the data stream from the Throttle Position Sensor and/or Pedal Position Sensor. Begin this inspection with the key "on" and the engine "off" and read the voltage output from the sensors at the resting or idle position. Then, slowly increase the amount of pedal and/or Throttle opening. The voltage should rise very evenly and without any glitches or drop offs throughout the entire range. On some "Drive by Wire" systems, you may have to start the engine to observe how the well the Throttle Position Sensor mirrors the voltage output of the Pedal Position Sensor. It might be necessary to drive the vehicle on a roadway to do a full range, data stream inspection of the sensors.

If You Can Verify a Glitch in Voltage Output

If you can verify that the Throttle/Pedal Position Sensor voltage "drops out" or goes out of range, conduct a visual inspection of the sensors to verify that the terminals are properly in their connectors. Next, do a "wiggle test"—look at the data stream with the key "on" and engine "off" to see if wiggling the wiring and connector has any effect. If if does, change the voltages and perform the appropriate wiring repair to the circuits in question. If the voltage readings are not effected by the "wiggle test," then replacing the sensor(s) may be the recommended next procedure.

If You Cannot Verify a Glitch in Voltage Output

If you cannot verify that the Throttle/Pedal Position Sensor has a "glitch" in its voltage output with the scan tool data stream, then deploy a lab scope to perform another sweep test and see if the more sensitive lab scope can detect a problem in the signal—a drop out or jagged signal that has "sharks' teeth" as opposed to a smooth line from closed to wide open throttle.

If the lab scope inspection yields no useful information, then you may try clearing the code and driving the vehicle to see if the code re-sets. If it does, recommend replacing the sensor. If the code does not set and this is for an emissions repair, replace the sensor to prevent any intermittent problems from occurring in the future with the Throttle/Pedal Position Sensor, such as unwanted acceleration or higher emissions output due to a worn Throttle/Pedal Position Sensor.

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