P2120 – Accelerator pedal position (APP) / Throttle position (TP) sensor/ switch D – circuit malfunction

By Reinier (Contact Me)
Last Updated 2020-08-12
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P2120 Mean?
2. Where is the P2120 sensor located?
3. What are the common causes of code P2120?
4. What are the symptoms of code P2120?
5. Get Help with P2120

What Does Code P2120 Mean?

OBD II fault code P2120 is a generic trouble code that is defined as “Throttle/Pedal Position Sensor/Switch “D” Circuit”, and is set when the PCM (Powertrain Control Module) detects an abnormally high or low voltage in the throttle pedal position sensor’s circuit labeled “D”. Note that labeling conventions differ between manufacturers, and while circuit “D” typically refers to the throttle pedal position sensor’s signal circuit, some manufacturers may assign a different label to this circuit. Consult reliable service information to confirm the function of circuit “D” on the affected vehicle.

NOTE: The throttle pedal position sensor should not be confused with the throttle position sensor. The former is attached to the throttle pedal to measure both the range and rate of movement of the throttle pedal, while the latter monitors the position of the throttle blade in the throttle body.

NOTE #2: Note that on some vehicles that use conventional throttle control systems, i.e., control cables, code P2120 will set when the PCM detects an abnormal and/or unexpected voltage in the signal voltage from the throttle position sensor in the throttle body.

On vehicles that use drive-by-wire throttle control systems, the throttle pedal is connected to a sensor that changes a 5-volt reference voltage as the pedal moves. As a practical matter, these sensors are simple potentiometers that typically allow between about 2.5 volts and about 0.5 volts to return to the PCM when the throttle pedal is in the rest position. As the throttle pedal moves, progressively more voltage passes back to the PCM as the electrical resistance in the sensor decreases, and at full throttle, the sensor passes about 4.5 volts back to the PCM.

These voltages are known as signal voltages, and the PCM uses them to activate a stepper motor in the throttle body to open and close the throttle blade. In a fully functional throttle control system, the position of the throttle pedal will correlate with the position of the throttle blade. To confirm the correlation, the PCM compares the position of the throttle blade with that of the throttle pedal continuously via a dedicated position sensor in the throttle body, but on some vehicles, the PCM will also monitor changes in the intake manifold pressure as a sort of “double-check” to confirm movements of the throttle pedal.

Thus, when the PCM detects a signal voltage from the throttle position sensor that differs from the expected value based on correlation checks, it will recognize that it can neither correlate the positions of the throttle pedal and the throttle blade effectively nor control the throttle effectively and safely. When this happens, the PCM will set code P2120 and illuminate a warning light, but note that the PCM will almost certainly also initiate a fail-safe or limp mode that may vary from limiting the engine speed severely, to immobilizing the vehicle completely.

Where is the P2120 sensor located?

This image shows the location of a throttle pedal position sensor (circled) relative to the actual throttle pedal (arrowed). Note that on some designs, the sensor is a separate component, and can be removed/replaced without removing the entire assembly. In this particular example though, the sensor is incorporated into the pedal, and the entire assembly must, therefore, be replaced should the need for replacement arise.

What are the common causes of code P2120?

Typical causes of code P2120 are much the same across all applications regardless of design, and could include one or more of the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
• Damaged, defective, or malfunctioning sensor
• Excessive mechanical wear of moving parts in the sensor
• Failure to perform a prescribed relearning procedure after a throttle position sensor replacement
• Defective or malfunctioning Absolute Manifold Pressure sensor, but note that this only applies to applications that use data from the MAP sensor for correlation purposes- consult reliable service information on this point

What are the symptoms of code P2120?

Typical symptoms of code P2120 are largely similar across all applications, and could include one or more of the following-

• Stored trouble code and illuminated MIL (CHECK ENGINE) light
• Note that in some cases, multiple other codes, and particularly codes relating to throttle pedal position/throttle position correlation could be present along with P2120
• Vehicle will almost certainly be in a fail-safe or limp mode that will persist until the fault is corrected but note that in some cases, the PCM may disable all throttle inputs as a safety precaution to prevent the vehicle from being driven

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