OBDII Code P2138: Understanding Throttle/Pedal Position Sensor Voltage Correlation

Decoding the P2138 Trouble Code: Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation

The OBDII trouble code P2138 is categorized as a generic powertrain code, meaning it’s applicable across a wide range of vehicle makes and models manufactured from 1996 onwards. While the code definition remains consistent, specific diagnostic and repair procedures can vary slightly depending on the vehicle’s manufacturer and model.

At its core, the P2138 code signals a voltage correlation issue within the throttle/pedal position sensor system. This system is crucial for regulating your vehicle’s engine power output based on your accelerator pedal input. To fully grasp the P2138 code, it’s essential to understand the evolution of throttle control systems in automobiles.

In older vehicles, a direct mechanical linkage, typically a cable, connected the accelerator pedal to the throttle body. The throttle body regulates the amount of air entering the engine, thereby controlling engine speed and power. A Throttle Position Sensor (TPS) was mounted on the throttle body to monitor the throttle plate’s angle. This sensor converted the mechanical throttle position into an electrical voltage signal, which was then relayed to the engine control unit (ECU) or engine management computer. The ECU used this voltage signal to determine the appropriate fuel injection and ignition timing.

However, modern vehicles have largely transitioned to “drive-by-wire” or Electronic Throttle Control (ETC) systems. This advancement was driven by the need for more precise control over various vehicle functions, including emissions, fuel efficiency, traction control, stability control, cruise control, and transmission behavior.

Electronic Throttle Control replaces the mechanical cable with an intricate system comprised of:

• Accelerator Pedal Position Sensor (APPS): This sensor, located at the accelerator pedal, detects the driver’s intended throttle input.
• Electronic Throttle Body: This unit houses an electric motor responsible for physically opening and closing the throttle plate. It also incorporates two embedded Throttle Position Sensors (TPS) for redundancy and accurate position monitoring.
• Engine Management Computer (ECU/PCM): The brain of the system, the ECU receives signals from the APPS and TPS sensors and controls the throttle body motor to achieve the desired throttle opening.

An electronic throttle body with the Throttle Position Sensor (TPS) highlighted as the black component on the lower right.

When you press the accelerator pedal, the APPS sends a signal to the ECU indicating the desired throttle input. The ECU then commands the throttle body motor to adjust the throttle plate accordingly. The two TPS sensors within the throttle body play a critical role by providing feedback to the ECU about the actual throttle plate position. The ECU constantly monitors and compares the voltage signals from these two TPS sensors to ensure they correlate correctly.

The P2138 code is triggered when the ECU detects a discrepancy or lack of correlation between the voltage signals from the two TPS sensors for a duration exceeding two seconds. This indicates a potential malfunction within the ETC system that could compromise throttle control, leading to potentially dangerous driving situations. While the P2138 code is fairly standardized, it’s worth noting that some manufacturers might use slightly different terminology, such as “Throttle position sensor circuit range/performance” (Infiniti) or “Electronic throttle control system malfunction power management” (Hyundai), to describe similar issues.

It’s also important to note that P2138 is closely related to other OBDII codes like P2135, P2136, P2137, P2139, and P2140. These codes often point to similar issues within the throttle position sensor system, and the diagnostic approach for these codes is generally consistent.

Symptoms of a P2138 Code

The symptoms associated with a P2138 code can vary in severity, but they all point to irregularities in throttle control. You might experience one or more of the following:

• Check Engine Light: The most common and immediate symptom is the illumination of the Check Engine Light on your dashboard.
• Stalling: The engine may stall, particularly when decelerating or coming to a stop.
• Reduced Engine Power: A noticeable lack of power, sluggish acceleration, or an inability to accelerate effectively.
• No Acceleration: In some cases, the vehicle may fail to accelerate at all when the accelerator pedal is pressed.
• Sudden Power Loss: Unexpected and abrupt loss of engine power while driving at cruising speeds.
• Stuck Throttle: The throttle might become stuck at a particular RPM, causing the engine to maintain a constant speed even when the accelerator pedal is released.

These symptoms highlight the critical nature of the throttle control system and the potential safety implications of a P2138 code. Addressing this issue promptly is crucial for safe vehicle operation.

Potential Causes of the P2138 DTC

Several factors can contribute to a P2138 trouble code. Based on practical experience, common causes include:

• Faulty Throttle Body Connector/Wiring: The electrical connector or “pigtail” at the throttle body is a frequent source of problems. Terminals within the connector can corrode, become loose, or even pull out, leading to poor electrical connections.
• Wiring Issues: Bare wires within the throttle body pigtail can short circuit to ground, disrupting the sensor signals.
• Distorted Throttle Body Cover: The plastic top cover of the throttle body, which houses the throttle plate actuator gears, can become distorted due to heat or physical damage. This distortion can impede the smooth movement of the gears and throttle plate.
• Defective Electronic Throttle Body: The throttle body itself, including the internal motor and TPS sensors, can fail.
• Faulty Accelerator Pedal Sensor (APPS): The accelerator pedal sensor or its associated wiring can malfunction, sending incorrect signals to the ECU.

An accelerator pedal assembly illustrating the integrated sensor and wiring responsible for transmitting pedal input.

• Engine Management Computer (ECU) Failure: Although less common, a failure within the ECU itself can lead to incorrect processing of sensor signals and trigger a P2138 code.
• TPS Sensor Miscorrelation/Relearn Requirement: In some instances, temporary discrepancies between the TPS sensors might set the P2138 code. The ECU may require a “relearn” procedure to recalibrate and restore proper throttle response. In rare cases, ECU reprogramming at a dealership might be necessary.

Diagnostic and Repair Steps for P2138

Diagnosing and repairing a P2138 code requires a systematic approach. Given the sensitivity of electronic throttle control systems, caution and careful handling of components are paramount.

1. Technical Service Bulletins (TSBs): Begin by consulting online resources or a service manual for Technical Service Bulletins (TSBs) related to your specific vehicle model and the P2138 code. TSBs are issued by manufacturers to address known issues and provide recommended repair procedures.

2. ECU Relearn Procedure: Research if your vehicle has a specific ECU relearn procedure for the throttle system. The original article provides an example for Nissan vehicles. Performing a relearn procedure might resolve the issue if the code was triggered by a temporary sensor miscorrelation.

3. Throttle Body Connector Inspection:

   • Disconnect the electrical connector from the throttle body.
   • Thoroughly inspect the connector terminals for corrosion, bending, looseness, or damage.
   • Clean any corrosion using a small precision screwdriver or electrical contact cleaner.
   • Apply a small amount of dielectric grease to the terminals to prevent future corrosion and improve connectivity.
   • Reconnect the connector securely.

4. Wiring Pigtail Replacement: If you find damaged or corroded terminals in the connector, consider replacing the wiring pigtail. These pigtails are generally available at auto parts stores or dealerships.

5. Throttle Body Cover Inspection: Examine the plastic top cover of the throttle body for any signs of cracks, warping, or distortion. If damage is evident, contact your dealer to inquire if the top cover is available as a separate replacement part. If not, throttle body replacement will be necessary.

6. Accelerator Pedal Sensor (APPS) Testing:

   • Use a voltmeter to test the APPS. Identify the 5-volt reference wire and the signal wire (refer to a wiring diagram for your vehicle).
   • With the ignition key turned to the “ON” position (engine off), probe the APPS signal wire.
   • Slowly depress the accelerator pedal from idle to wide-open throttle.
   • The voltage reading on the signal wire should increase smoothly from approximately 0.5 volts to 5.0 volts without any spikes, drops, or irregularities.
   • If the voltage signal is erratic or absent, replace the APPS.

7. Throttle Body Motor Power Test:

   • Identify the power and ground terminals for the throttle body motor in the connector (refer to a wiring diagram).
   • Use a voltmeter to probe these terminals.
   • Have an assistant turn the ignition key to the “ON” position and slightly depress the accelerator pedal.
   • You should measure voltage at the throttle body motor connector.
   • If voltage is present, suspect a faulty throttle body.
   • If voltage is absent, the issue might lie with the ECU or associated wiring.

Disclaimer: This information is for informational purposes only and should not be considered professional repair advice. Automotive repair procedures should be performed by qualified technicians. We are not liable for any consequences resulting from actions taken based on this information.

For further assistance with a P2138 code, consider seeking advice from certified mechanics or utilizing online automotive repair forums for community support and troubleshooting tips.

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Related DTCs: P0068, P0120, P0121, P0122, P0123, P0124, P0510.