The P2138 trouble code is an OBD-II diagnostic code that indicates a problem with the throttle or accelerator pedal position sensor circuit, often affecting vehicle acceleration and performance. This article provides a detailed analysis of P2138, covering its definition, causes, symptoms, diagnostic steps, and solutions. Supported by real-world case studies—such as issues in the Chevrolet Malibu and other models—this guide offers practical insights for vehicle owners and technicians to effectively resolve this fault.

1. What is P2138?

P2138 is an OBD-II diagnostic trouble code (DTC) defined as "Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation." It indicates that the Engine Control Module (ECM) has detected a mismatch between the voltage signals from the throttle position sensor (TPS) or accelerator pedal position (APP) sensor circuits, specifically between the D and E circuits. These sensors are critical for the drive-by-wire system, which controls throttle operation based on driver input.

Role of the Throttle/Pedal Position Sensors

The APP sensor, located at the accelerator pedal, and the TPS, located at the throttle body, work together to inform the ECM of the driver’s throttle demand and the actual throttle position. The D and E circuits are redundant signals for safety, ensuring accurate throttle control. A voltage correlation error means the ECM cannot trust the signals, triggering P2138 and often activating a failsafe mode.

Common Trigger Conditions

• Voltage signals from the D and E circuits of the TPS or APP sensor do not correlate.
• Electrical issues in the sensor circuits.
• Mechanical or sensor failures affecting signal accuracy.

2. Potential Causes and Specific Issues of P2138

P2138 can stem from various factors. Below is a detailed list of common causes, supported by real-world examples:

• (1) Faulty Accelerator Pedal Position (APP) Sensor
  Issue: The APP sensor is defective, sending inconsistent signals between its D and E circuits.
  Example: In a 2016 Chevrolet Malibu, the APP sensor failed, causing a voltage mismatch between D and E circuits and triggering P2138.
  Cause: Wear from prolonged use or internal sensor failure.
• (2) Faulty Throttle Position Sensor (TPS)
  Issue: The TPS provides conflicting voltage signals, leading to a correlation error.
  Example: A 2017 Ford Fusion exhibited P2138 due to a worn TPS, resulting in erratic throttle response.
  Cause: Contamination or mechanical wear in the throttle body.
• (3) Wiring or Connector Issues
  Issue: Damaged, shorted, or corroded wiring in the TPS or APP sensor circuits causes signal discrepancies.
  Example: A 2018 Toyota Camry triggered P2138 because of a corroded connector in the APP sensor circuit, affecting signal integrity.
  Cause: Exposure to moisture or vibration-induced wear.
• (4) Throttle Body Malfunction
  Issue: A dirty or malfunctioning throttle body affects TPS readings, leading to correlation errors.
  Example: A 2015 Honda Accord had P2138 due to a sticky throttle body, causing inconsistent TPS signals.
  Cause: Carbon buildup or mechanical failure in the throttle body.
• (5) ECM Software or Hardware Issues
  Issue: The ECM misinterprets sensor signals or has an internal fault.
  Example: A 2019 Nissan Altima showed P2138 due to an ECM software glitch, resolved with a firmware update.
  Cause: Outdated software or internal ECM failure.

3. Symptoms of P2138

Symptoms of P2138 are often related to throttle control and vehicle performance. Here are common signs and their associated causes:

• Check Engine Light On
  Symptom: MIL illuminates on the dashboard.
  Cause: ECM detects a voltage correlation error.
• Reduced Engine Power (Limp Mode)
  Symptom: Vehicle enters a reduced power mode with limited acceleration.
  Cause: ECM activates failsafe mode to prevent unsafe operation.
• Unresponsive or Erratic Throttle
  Symptom: Throttle response is delayed, jerky, or inconsistent.
  Cause: Conflicting signals from the TPS or APP sensor.
• Engine Stalling or Hesitation
  Symptom: Engine stalls or hesitates during acceleration.
  Cause: ECM cannot accurately control the throttle.
• Poor Acceleration
  Symptom: Sluggish response when pressing the accelerator.
  Cause: Throttle not opening as commanded due to signal mismatch.

4. Vehicles Prone to P2138 and Case Studies

P2138 is common in vehicles with drive-by-wire throttle systems. Below are notable examples:

• Chevrolet Malibu (2016-2020)
  Issue: Faulty APP sensor.
  Symptom: Limp mode and poor acceleration.
• Ford Fusion (2013-2018)
  Issue: Worn TPS.
  Symptom: Erratic throttle response and MIL on.
• Toyota Camry (2012-2018)
  Issue: Corroded APP sensor connector.
  Symptom: Unresponsive throttle and stalling.
• Honda Accord (2013-2017)
  Issue: Dirty throttle body.
  Symptom: Hesitation and reduced power.
• Nissan Altima (2013-2019)
  Issue: ECM software glitch.
  Symptom: MIL on with intermittent throttle issues.

5. Steps to Diagnose P2138

Diagnosing P2138 requires a systematic approach. Follow these steps:

Step 1: Read Trouble Codes and Data Stream
Tool: OBD-II scanner (e.g., Launch X431).
Action: Connect to the OBD-II port, retrieve P2138 and related codes, and monitor live data.
Key Data:

1. 1. APP sensor D/E voltages: Should correlate (e.g., 0.5-4.5V, proportional to pedal position).
   2. TPS D/E voltages: Should match throttle position (e.g., 0.5-4.5V).

Example: The Malibu showed a 1V difference between APP D and E circuits.

Step 2: Visual Inspection
Checkpoints:

1. 1. APP sensor and connector: Look for damage or corrosion.
   2. TPS and throttle body: Check for dirt or mechanical issues.
   3. Wiring harness: Inspect for shorts, frays, or loose connections.

Example: The Camry had a corroded pin in the APP sensor connector.

Step 3: Test the APP Sensor
Tool: Multimeter.
Action:

1. 1. Measure D/E circuit voltages: Should increase smoothly with pedal movement.
   2. Check resistance: Should be within specs (e.g., 1-5 kΩ).

Example: The Malibu’s APP sensor showed inconsistent voltages.

Step 4: Test the TPS
Tool: Multimeter.
Action:

1. 1. Measure D/E circuit voltages: Should increase with throttle opening.
   2. Check for smooth operation: No sudden jumps in voltage.

Example: The Fusion’s TPS had erratic voltage changes.

Step 5: Inspect the Throttle Body
Action:

1. 1. Clean the throttle body and retest TPS signals.
   2. Check for mechanical binding or sticking.

Example: The Accord’s throttle body was sticky, affecting TPS readings.

Step 6: Verify the ECM
Action:

1. 1. Test with a known-good APP sensor or TPS.
   2. Update ECM firmware if applicable.

Example: The Altima’s issue resolved after a software update.

6. Solutions and Implementation

Apply these repairs based on diagnostic findings:

• Replace the APP Sensor
  Part: Chevrolet OEM APP sensor (~$100).
  Steps: Disconnect the connector, remove the pedal assembly, install a new sensor, and clear codes.
  Example: The Malibu’s throttle response normalized post-replacement.
• Replace the TPS or Throttle Body
  Part: Ford OEM throttle body (~$150).
  Example: The Fusion’s erratic throttle fixed after replacement.
• Repair Wiring or Connectors
  Method: Clean corrosion or replace damaged connectors.
  Example: The Camry’s connector repair restored signal integrity.
• Clean the Throttle Body
  Part: Throttle body cleaner (~$10).
  Example: The Accord’s hesitation stopped after cleaning.
• Update ECM Software
  Tool: Dealer equipment.
  Example: The Altima’s P2138 cleared after the update.

7. Repair Costs and Precautions

• APP Sensor: $100-$200.
• Throttle Body/TPS: $150-$300.
• Wiring Repair: $50-$150.
• Throttle Body Cleaning: $10-$50.
• ECM Update: $0-$150.

Precautions

• Perform an idle relearn procedure after throttle body repairs.
• Use dielectric grease on connectors to prevent corrosion.

8. Preventive Measures

• Clean the throttle body every 30,000 miles.
• Inspect wiring harnesses during routine maintenance.
• Avoid exposing the engine bay to excessive moisture.
• Keep ECM software updated with manufacturer releases.

Summary

The P2138 trouble code indicates a voltage correlation error between the D and E circuits of the throttle or pedal position sensors, often due to faulty sensors, wiring issues, or throttle body problems. Through detailed case studies—like the APP sensor failure in the Chevrolet Malibu or the dirty throttle body in the Honda Accord—and systematic diagnostics, this guide equips readers to diagnose and fix the problem efficiently. Addressing P2138 promptly restores throttle response and prevents unsafe driving conditions. For thorough diagnostics, tools like the iCarsoft CR MAX from www.icarsoft-us.com provide real-time data and code-clearing capabilities to maintain optimal vehicle performance.