P2121 OBD-II Trouble Code: Throttle/Pedal Position Sensor/Switch D Circuit Range/Performance

P2121 code definition

The powertrain control module (PCM) or another related control module has detected a problem in a particular throttle position sensor circuit. Related modules can include:

• Transmission control module
• Body control module
• Antilock brake control module
• Instrument panel control module
• Fuel injection control module
• Turbo control module
• Anti-theft module
• Cruise control module
• Traction control module
• Proximity alert module
• Climate control module

What the P2121 code means

If the throttle actuator control motor circuit sends a reading to the PCM that is out of normal specs, a trouble code will be stored and the malfunction indicator lamp (MIL) will illuminate. Some models may require several drive cycles with the failure to illuminate the MIL, while others will activate it immediately.

What causes the P2121 code?

• Defective throttle actuator control motor
• Defective throttle position actuator
• Defective throttle position sensor or pedal position sensor
• Corroded, open or shorted wiring or connectors in throttle actuator motor or Controller Area Network (CAN) bus harness
• Loose control module ground strap
• Broken ground wire

What are the symptoms of the P2121 code?

• Black smoke at startup
• Poor acceleration
• Misfire at idle
• Forced shutdown
• Forced idle
• Start/stall condition
• No-start condition

How does a mechanic diagnose the P2121 code?

An OBD-II scanner/code reader, digital volt/ohmmeter (DVOM), oscilloscope and manufacturer’s service manual will be necessary for this diagnosis and repair. NOTE: P2121 codes only apply to vehicles with an electronically controlled “drive by wire” throttle body. These are designed with a sensor on the accelerator pedal arm which sends a voltage signal to the PCM or other related controller. This is what commands the throttle body to open or close, depending on conditions. The PCM enables the throttle actuator motor in the required direction by changing voltage and polarity. As the throttle actuator turns, it turns a gear which meshes with a larger gear, allowing the throttle plate to open quickly and suddenly or slowly, depending on the driver input.

• Begin with a visual inspection of all wiring and connectors.
• Repair/replace any damaged, disconnected, shorted or corroded wiring, connectors and components as needed. Retest the system after repairs are completed.
• If all system wiring, connectors and components are in good condition, connect the scanner/code reader to the diagnostic connector. Record any stored codes and freeze frame the data. Clear the codes and test drive the vehicle to see if codes return. Remember that on some vehicles, multiple drive cycles may be needed for the MIL to illuminate.
• Test the throttle position sensor electrical connector by disconnecting it from the sensor. Use the DVOM to test for voltage and ground signals. A 5-volt reference signal and a battery ground should be available. The manufacturer’s service manual will show exact wiring diagrams and connector views.
• If no reference voltage and/or ground are found, disconnect the connector at the PCM. Test the any related controller connectors for resistance and continuity. Repair/replace shorted/open circuits as needed.
• Clear codes and test drive the vehicle.
• If voltage/ground signals are present at the connector for the throttle actuator motor, replace the motor, clear codes and drive the vehicle.
• In many cases, the entire throttle body will need to be replaced, including the actuator motor and throttle position sensor.
• Pedal sensor replacement may also be necessary with throttle body replacement.
• If voltage/ground signals are within manufacturer’s specs, use the oscilloscope to monitor pedal sensor operation, looking for abnormal spikes in voltage or other glitches.

Common mistakes when diagnosing the P2121 code

Often, technicians treat symptoms rather than the cause of a problem when multiple codes are registered. Codes related to lean exhaust, engine misfires, fuel injectors or numerous other codes can be easy to misdiagnosis when accompanied by a controller communication code. Remember to diagnose the codes in the order in which they were stored, to help your chances for a proper diagnosis and repair. Using the scanner’s freeze frame function can help determine the order in which codes were stored.

How serious is the P2121 code?

A P2121 code can result in symptoms that make the vehicle undrivable.

What repairs can fix the P2121 code?

• Throttle body replacement
• Throttle actuator motor replacement
• Pedal position sensor replacement

Additional comments for consideration regarding the P2121 code

On post-2008 vehicles, a code such as P2121 may be traced to the CAN bus. The CAN bus is a conglomeration of sensors, wiring harnesses and connectors that enables information to be shared between modules, and it controls almost every electrical function of the vehicle. A technician with a specialized scanner such as Autohex or Tech II can determine CAN-related issues, but it may be something that’s very difficult to diagnose and repair for a non-professional mechanic. Probing each individual circuit, out of hundreds, could result in hours and hours of labor.

Proper service of CAN networks requires a hard drive to retain memory in case of a complete system meltdown; all it can take is one misplaced DVOM probe to cause a catastrophe. A professional technician with a specialized CAN scanner, on the other hand, can determine pin values and control module operation without the risk of a system meltdown. Keep an eye open for engine or transmission ground cables, straps or wires; sometimes these are left dangling after recent repairs.

In many cases, problems like a P2121 are simply beyond the expertise of a non-professional mechanic and should be left to a qualified technician. Sophisticated CAN systems are designed for service by factory-trained technicians and not non-professionals.

Need help with a P2121 code?

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