P0121 OBD-II: Throttle/Pedal Position Sensor/Switch "A" Circuit

What Does P0121 Code Mean?

OBD-II Code P0121 is defined as a Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance Problem

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 P0121 triggers when the Throttle/Pedal Sensor/Switch "A" Circuit sends irrational data to the PowerTrain Control Module or PCM.

P0121 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 P0121 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

P0121 Diagnostic Theory for Shops and Technicians

When diagnosing a P0121 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 performance 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.