As an auto repair expert at obd-de.com, I’m here to break down the OBD2 trouble code P2138. This code, known as Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation, signals an issue within your vehicle’s electronic throttle control system. If you’re experiencing engine performance problems and this code has popped up, understanding its causes, symptoms, and fixes is crucial. Let’s dive into what P2138 means and how to address it.
Decoding P2138: Throttle/Pedal Position Sensor Voltage Mismatch
The P2138 diagnostic trouble code is a generic powertrain code, meaning it can occur in most vehicles manufactured from 1996 onwards. While the general meaning is consistent across car brands, specific repair steps can vary slightly depending on your vehicle’s make and model.
This code points to a problem in the electronic throttle control system, often referred to as “drive-by-wire.” Modern vehicles transitioned to this technology in the 1990s to enhance control over emissions, fuel efficiency, traction, stability, cruise control, and transmission behavior.
Before “drive-by-wire,” a simple cable directly linked the gas pedal to the throttle body, controlling engine speed. A Throttle Position Sensor (TPS) on the throttle body converted throttle movement into a voltage signal for the engine computer.
Electronic throttle control replaces this cable with sensors and actuators. It includes:
- Accelerator Pedal Position Sensor (APPS): Measures how much you press the gas pedal.
- Electronic Throttle Body: Contains a motor to open and close the throttle plate electronically.
- Dual Throttle Position Sensors (TPS): Integrated within the throttle body to provide redundant feedback on throttle plate position.
- Engine Management Computer (PCM/ECM): The brain of the system, processing sensor inputs and controlling the throttle.
While the terminology can differ slightly by manufacturer (e.g., “Throttle position sensor circuit range/performance” on Infiniti or “Electronic throttle control system malfunction power management” on Hyundai), the core issue remains the same.
When you press the accelerator, the APPS signals your desired throttle opening to the computer. The computer then commands the throttle body motor to adjust the throttle plate accordingly. The two embedded TPS sensors continuously report the throttle plate’s actual position back to the computer as voltage signals.
Throttle body with Throttle Position Sensor (TPS) highlighted. The TPS is a black component on the lower right of the throttle body.
The computer constantly monitors and compares the voltage signals from both TPS sensors. P2138 is triggered when these voltage signals become inconsistent or don’t correlate for a sustained period (typically two seconds). This discrepancy indicates a potential malfunction within the electronic throttle control system, which can lead to serious driveability issues. It’s important to note that P2138 may sometimes be accompanied by other trouble codes, providing more specific clues about the underlying problem. Loss of throttle control is a serious safety concern, making prompt diagnosis and repair essential.
Here’s a look at the accelerator pedal and its sensor:
Accelerator pedal with attached sensor and wiring. The sensor is integrated into the pedal assembly and wiring harness connects to it.
Important Note: P2138 shares diagnostic similarities with other related codes such as P2135, P2136, P2137, P2139, and P2140. The diagnostic approach for these codes will generally be similar.
P2138 Code Symptoms: What You Might Notice
When the P2138 code is active, you may experience a range of symptoms affecting your vehicle’s performance. These can include:
- Stalling: The engine may stall, especially when decelerating or coming to a stop.
- Lack of Power/No Acceleration: The engine may feel sluggish, unresponsive, or completely fail to accelerate when you press the gas pedal.
- Sudden Power Loss: You might experience unexpected and abrupt loss of engine power while driving at cruising speeds.
- Stuck Throttle: In rare cases, the throttle might become stuck at a certain RPM, causing the engine to maintain speed even when you lift your foot off the accelerator.
- Check Engine Light: The malfunction indicator lamp (MIL), or check engine light, will illuminate on your dashboard.
Common Causes of P2138: Pinpointing the Problem
Several potential issues can lead to a P2138 code. Based on my experience in auto repair, here are some of the most frequent causes:
- Faulty Throttle Body Connector/Wiring: A common culprit is the wiring connector (pigtail) at the throttle body. Terminals within the connector can corrode, loosen, or become damaged, leading to poor electrical connections.
- Bare Wire Shorting: The wiring in the throttle body pigtail might have insulation damage, causing a bare wire to short circuit to ground.
- Distorted Throttle Body Cover: The plastic top cover on the throttle body can sometimes warp or become distorted. This distortion can interfere with the internal gears that control the throttle plate movement, affecting sensor readings.
- Defective Electronic Throttle Body: The throttle body itself, including its internal motor and TPS sensors, can fail.
- Failed Accelerator Pedal Sensor (APPS): Problems with the accelerator pedal sensor or its associated wiring can also trigger P2138.
- Engine Computer (PCM/ECM) Malfunction: In rare cases, a fault within the engine management computer itself could be the cause.
- TPS Relearn/Reprogramming Needed: Sometimes, the computer loses its learned throttle position parameters. This can happen after repairs or battery disconnection. A throttle position sensor relearn procedure or computer reprogramming at the dealership might be necessary.
Diagnosing and Repairing P2138: Step-by-Step Guide
The electronic throttle control system is delicate and sensitive to damage. Handle components with extreme care during diagnosis and repair. Avoid dropping or rough handling, as this could lead to further issues.
Here’s a step-by-step approach to diagnosing and repairing a P2138 code:
1. Check for Technical Service Bulletins (TSBs):
- Begin by searching online for TSBs related to P2138 and your specific vehicle (year, make, and model). TSBs are issued by manufacturers to address known issues and provide recommended repair procedures. They can offer valuable insights and shortcuts for diagnosis.
2. Attempt a Throttle Relearn Procedure:
- Consult your vehicle’s service manual or online resources for a potential throttle relearn procedure. This process resets the computer’s throttle position memory. The Nissan example provided in the original article gives an idea of what these procedures can entail, but always use the procedure specific to your vehicle.
3. Inspect the Throttle Body Connector and Wiring:
- Disconnect the electrical connector from the throttle body.
- Carefully examine the connector terminals for:
- Corrosion: Look for any signs of green or white powdery buildup. Clean corrosion with a small screwdriver or electrical contact cleaner.
- Bent or Missing Terminals: Check for bent or broken pins or terminals that have been pushed out of the connector.
- Loose Terminals: Ensure all terminals are firmly seated in the connector.
- Apply a small amount of dielectric grease to the terminals to prevent future corrosion and improve electrical contact. Reconnect the connector securely.
- If you find damaged terminals or wiring, a replacement “pigtail” connector can often be purchased from auto parts stores or the dealership.
4. Examine the Throttle Body Top Cover:
- Inspect the plastic top cover of the throttle body for any signs of cracks, warping, or distortion. If damage is present, inquire at the dealership about purchasing a replacement cover. If a cover is not available separately, throttle body replacement may be necessary.
5. Test the Accelerator Pedal Position Sensor (APPS):
- Use a voltmeter to test the APPS. You should typically find a 5-volt reference voltage and a signal wire.
- With the ignition key in the “ON” position (engine off), slowly depress the accelerator pedal.
- Monitor the voltage signal on the signal wire. It should smoothly increase from around 0.5 volts at idle to approximately 5.0 volts at wide-open throttle.
- If the voltage signal is erratic, spikes, drops out, or is absent, the APPS may be faulty and require replacement.
6. Check for Power to the Throttle Body Motor:
- Identify the wire terminals on the throttle body connector that supply power to the throttle motor. You may need to consult a wiring diagram for your specific vehicle.
- Use a voltmeter to probe these terminals.
- Have a helper turn the ignition key to the “ON” position and slightly depress the accelerator pedal.
- You should observe voltage at the motor terminals when the pedal is pressed.
- No Power: If no power is present, suspect a problem with the engine computer or wiring between the computer and throttle body.
- Power Present: If power is reaching the throttle body but the problem persists, the throttle body itself is likely malfunctioning and needs replacement.
Important Note: These diagnostic steps require basic automotive electrical testing knowledge and tools. If you are not comfortable performing these tests, it’s best to seek assistance from a qualified automotive technician.
Related DTCs: You might also encounter other throttle-related DTCs alongside P2138, such as P0068, P0120, P0121, P0122, P0123, P0124, P0510, and others. Understanding these codes can provide a broader picture of potential issues within the throttle control system.
Need More P2138 Help?
If you’re still struggling to diagnose or repair the P2138 code, our free car repair forums are a great resource. Post your question in our forums to connect with experienced mechanics and get personalized advice for your specific situation.
Disclaimer: This information is for informational purposes only and should not be considered professional repair advice. We are not liable for any actions taken based on this information. Always consult a qualified automotive technician for diagnosis and repair.
