The P2138 code signals a correlation fault between two accelerator pedal position sensors (and in some cases the associated throttle position sensor). In practice, the ECU detects that the pedal/ throttle signals disagree beyond the allowed tolerance, which can trigger reduced power or limp-home mode and illuminate the check engine light.
What P2138 Indicates
In many modern vehicles, two independent sensors monitor the pedal position (and sometimes the throttle position) and the engine control unit (ECU) compares their outputs in real time. A persistent mismatch beyond the manufacturer’s tolerance sets P2138, signaling that the two channels are not in agreement and the vehicle should not rely on full throttle control.
Keep in mind that different manufacturers may phrase this code slightly differently (for example, as “TPS/Pedal Position Sensor correlation” or “D/E channel correlation”). The core idea is the same: lack of reliable correlation between the two position sensors.
Understanding this helps frame the rest of the diagnosis: the issue is not simply a single sensor failure, but a disagreement between two sensing elements that the ECU uses to determine throttle opening.
Key points about P2138 include the following:
- The two position sensor outputs must stay within a defined tolerance across the pedal’s range.
- The ECU compares sensor A and sensor B signals; a persistent out-of-range difference triggers P2138.
- Related codes (varying by model) may include other throttle/pedal correlation codes such as P2135 or P2137.
Concluding that the correlation is off helps technicians focus on sensor health, wiring, or calibration rather than treating a random electrical fault as the sole cause.
Possible Causes
Several issues can lead to P2138, often in combination with other symptoms or codes:
- Faulty accelerator pedal position sensor(s) or throttle position sensor(s)
- Electrical interference or intermittent sensor signals caused by moisture or wiring routing
- ECU calibration or learning procedure not completed after sensor replacement
- Software updates or recalls affecting sensor synchronization
Diagnosing accurately requires considering vehicle make/model, as sensor layouts and naming can vary significantly.
Symptoms You Might Notice
Vehicles with a P2138 fault often exhibit drivability changes or warning indicators:
- Check Engine light (CEL) or Malfunction Indicator Light (MIL) on
- Reduced power, limp mode, or delayed throttle response
- Unstable idle or occasional stalling at idle
- Unexplained loss of acceleration or surges in throttle
- Intermittent tireless acceleration behavior depending on sensor correlation
If you experience these symptoms, a diagnostic check using an OBD-II scanner is advised to confirm P2138 and identify related codes.
Diagnostics and Repair
To diagnose and repair P2138, technicians typically follow a structured approach:
- Use an OBD-II scanner to read P2138 and any related codes (such as P2135 or P2137) and review freeze-frame data.
- Inspect wiring and connectors from the accelerator pedal module to the ECU and throttle body (if applicable) for damage, corrosion, and secure connections.
- Measure sensor outputs (voltage/resistance) from both pedal position sensors (and throttle position sensor if present) across the operating range, using manufacturer service procedures.
- Check for proper calibration or learn procedures; perform a pedal calibration or re-learn if the vehicle requires it after sensor replacement.
- Replace faulty components (pedal position sensor, throttle position sensor, or wiring harness) and clear codes; verify repair by road testing the vehicle.
- Be aware that some vehicles require software updates or dealer tooling to correct correlation logic.
Post-repair testing should confirm that sensor signals are within tolerance and that throttle response is consistent across the entire pedal range.
When to Seek Professional Help
Consider professional service if:
- The check engine light remains on after basic inspections and repairs
- Power loss or abnormal throttle behavior persists under varying driving conditions
- There are multiple related codes or intermittent symptoms that require specialized diagnostic tools
Professional diagnostics ensure that vehicle-specific calibration and software requirements are correctly followed to restore proper function.
Manufacturer Variations
Automakers differ in how they label pedal/throttle sensors and which exact codes correspond to correlation issues. The general principle remains: two independent sensors must stay coordinated within tolerance, and a failure to correlate triggers a code such as P2138. Always consult the OEM service information for your model for precise wiring diagrams, sensor tests, and calibration steps.
Summary
P2138 denotes a correlation fault between accelerator pedal position sensors (and, in some cases, throttle position sensors). It means the two sensor channels are not agreeing within the expected tolerance, which can lead to reduced power or limp mode and a check engine light. Diagnosing involves verifying sensor health, wiring integrity, and performing any required calibration or software updates. Addressing the issue promptly helps restore reliable throttle response and overall drivability.