The P2138 code signals a mismatch between the two accelerator pedal position sensors (APP A and APP B) feeding the engine control module in drive-by-wire systems, which can affect throttle response and may trigger a limp mode or a check engine light.
What P2138 Means
P2138 stands for Throttle/Pedal Position Sensor 1/2 Correlation. In modern vehicles with electronic throttle control, the accelerator pedal provides two position signals to the PCM. The PCM compares these signals across the pedal’s travel and expects them to stay within a defined tolerance. When the correlation between the two sensors falls outside that range, the code is recorded and the vehicle may restrict throttle to protect the drivetrain.
Common causes
Most P2138 cases come from hardware or wiring issues rather than a raw engine fault. Below are the typical culprits you’ll want to check first.
- Faulty accelerator pedal position sensor A or B (internal failure or miscalibration)
- Damaged, corroded, or loose wiring or connectors in the pedal harness or the PCM connector
- Water intrusion or contamination inside the pedal assembly
- ECU/PCM software fault or a need for a manufacturer software update
- Faulty throttle pedal module or defective pedal assembly
- Poor grounding or unstable power supply to the pedal sensors
In some vehicles, P2138 can appear alongside related codes (such as P2135 or P2139) that point to broader APP sensor correlation problems.
Symptoms you might notice
When APP sensor correlation is out of spec, drivers may notice changes in how the vehicle responds to throttle input or warning lights on the dash. Common symptoms include:
- Check Engine Light (MIL) illuminated
- Reduced power, limp mode, or delayed/throttled acceleration
- Unpredictable or inconsistent pedal response
- Occasional idle irregularities or engine stalling in rare cases
- Intermittent driveability issues that come and go
Note that some vehicles may run with the code stored and no obvious symptoms, especially if the issue is intermittent or the PCM has temporarily compensated.
Diagnosis and repair steps
Diagnosing P2138 involves verifying sensor readings, inspecting wiring, and confirming the integrity of the pedal assembly. The following steps outline a typical approach; always follow the vehicle’s service manual for exact procedures and safety precautions.
Before starting the diagnostic steps, connect a scan tool capable of live data to observe APP sensor A and B values while you operate the pedal.
- Observe live APP A and APP B readings across the full pedal travel to confirm whether the two sensors remain in proper correlation within the manufacturer’s tolerance.
- Check for related codes that could influence readings or mask the fault (such as code clusters tied to the throttle or pedal system).
- Visually inspect the accelerator pedal assembly, wiring harnesses, and connectors for damage, corrosion, moisture, or looseness.
- Test the sensor signals with appropriate diagnostic tools (multimeter or scope) if feasible, or rely on the scan tool’s live data to verify consistent tracking between A and B.
- Inspect power and ground circuits for the pedal sensors to ensure clean voltage and solid grounding.
- If wiring or connectors show damage, repair or replace as needed; if the sensors themselves appear faulty, replace the pedal position sensor(s) or the entire pedal assembly per OEM guidelines.
- After hardware replacement, perform any required calibration or learn procedures (pedal position learning) as specified by the manufacturer.
After completing diagnostics and any repairs, clear the codes, drive normally, and re-scan to ensure P2138 does not return. If the code reappears, re-evaluate wiring, sensors, and ECU software, as there may be a secondary issue or a need for an updated calibration.
Repair options
Based on the diagnostic findings, repairs generally fall into hardware replacement, wiring repair, or software calibration. The following options cover common remedies.
- Replace the accelerator pedal position sensor(s) or the entire pedal assembly (pedal module)
- Repair or replace damaged wiring, harnesses, and connectors; reseat and clean grounding points
- Update ECU/PCM software or apply manufacturer-recommended recalibration for APP sensors
- Perform pedal calibration or learn procedure after replacement to reestablish proper APP correlation
- Address any related drive-by-wire components if a separate fault exists (e.g., throttle body, actuator, or related control modules)
As with many drive-by-wire issues, OEM service bulletins or recalls may apply. If in doubt, consult your dealership or a qualified automotive technician for model-specific procedures and software updates.
When to seek professional help
P2138 can range from a simple sensor replacement to a more involved ECU re-learn. If you’re unsure about performing electrical tests or pedal calibration, or if the vehicle exhibits persistent loss of throttle control, seek a professional technician promptly to ensure safe driving conditions.
Summary
P2138 indicates a discrepancy between the two accelerator pedal position sensor readings (A and B) feeding the PCM. It is typically caused by faulty sensors, damaged wiring, compromised connectors, or software/calibration issues. Diagnosis centers on live data review, thorough wiring inspection, and, when needed, replacement of the pedal assembly with subsequent calibration. Proper resolution usually restores throttle response and clears the check engine light, but repeated occurrences should be evaluated by a technician to rule out ECU or systemic drive-by-wire faults.