Understanding the P2138 Throttle Position Sensor Code
The P2138 is a generic OBD-II trouble code that indicates a problem with the throttle/accelerator pedal position sensor circuit. This code is triggered when the engine control module (ECM) detects an abnormal voltage signal from the throttle position (TP) sensor and/or accelerator pedal position (APP) sensor that doesn't match the expected values.
In our workshop, we've seen the P2138 code pop up quite frequently, especially on European makes like Audi, BMW, and Volkswagen. Just last week, a customer brought in their 2015 Audi A4 with the check engine light on and complaining of poor acceleration. Sure enough, our scan tool pulled up a P2138 code. We traced it back to a faulty accelerator pedal position sensor. The wiring was corroded and causing intermittent signal issues. We replaced the sensor and cleaned up the wiring harness, which resolved the problem. It's a common issue on these cars, often due to wear and tear or exposure to moisture. While we occasionally see P2138 on Asian and American vehicles, it seems to be more prevalent on the German brands in our experience.
Urgency and Drivability
A P2138 code is considered a serious issue that requires immediate attention. When this fault is detected, the ECM will put the vehicle into "limp mode", limiting throttle response to less than 50% to prevent unintended acceleration. Driving the vehicle is not recommended as you may experience:
- Sudden loss of power
- Poor acceleration
- Engine stalling
- Inability to change engine speed if the throttle sticks
Take your vehicle to a qualified mechanic as soon as possible to diagnose and repair the issue.
Parts Involved
The main components related to a P2138 code are:
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- Accelerator Pedal Position (APP) Sensor – Measures accelerator pedal movement and sends a voltage signal to the ECM indicating driver input
- Throttle Position (TP) Sensor – Mounted on the throttle body and sends a voltage signal to the ECM indicating actual throttle plate angle
- Throttle Body – Controls airflow into the engine and contains the TP sensor
- Engine Control Module (ECM) – Monitors APP and TP sensor signals to control the throttle motor
The ECM expects the APP and TP sensor voltages to correlate with each other. If they deviate for more than 1-2 seconds, a P2138 is triggered.
Common Causes
Potential causes of a P2138 code include:
- Faulty APP sensor (most common)
- Faulty TP sensor
- Dirty or sticking throttle body
- Damaged wiring/connectors in the APP or TP circuits
- Corroded electrical connections
- Defective ECM (rare)
Diagnosis Steps
To properly diagnose a P2138 code:
- Scan for all codes and document freeze frame data
- Visually inspect the throttle body and APP sensor for damage or contamination
- Test APP and TP sensor reference voltages and ground signals
- Observe APP and TP sensor parameters with a scan tool while depressing the accelerator
- Perform a throttle body cleaning service
- Check for damaged wires or poor connections in the APP and TP circuits
- Replace the APP and/or TP sensor if faulty
An advanced scan tool with bi-directional control is very helpful for diagnosis. Some vehicles require an idle relearn procedure after sensor replacement.
Repair Costs
APP sensor replacement typically costs $120-$300 in parts and $80-$120 in labor (1.0 hours). A throttle body cleaning service runs $75-$150. Wiring repairs can vary widely from $50 to over $500 depending on the extent of the damage.
In rare cases, a faulty ECM may need replacement at a cost of $800-$1500 (€680-€1275). Exhaust all other diagnostic possibilities before condemning the ECM.
Prevention and Related Issues
Keeping the throttle body clean by using a quality fuel system cleaner every 30,000 km (18,000 miles) can help prevent APP and TP sensor issues. Inspect electrical connectors regularly and avoid spilling liquids in the engine compartment.
Other codes that may appear along with P2138 include P2135, P2122, P2127, P2101, and P0121 – all related to the electronic throttle control system. Address these codes as a system, not individually.
By understanding the P2138 code's causes, symptoms, and diagnostic steps, you can quickly resolve this issue and get your vehicle running properly again. If you're unsure about your diagnosis, consult with a professional technician for further assistance.
Frequently Asked Questions about P2138 Code
What does the P2138 code mean?
The P2138 is a generic OBD-II trouble code that stands for "Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation." It indicates that the engine control module (ECM) has detected a discrepancy between the signals from the throttle position (TP) sensors and/or accelerator pedal position (APP) sensors.
What are the symptoms of the P2138 code?
Common symptoms of P2138 include:
- Check engine light illumination
- Sudden loss of power at cruising speeds
- Poor acceleration or unresponsive engine
- Engine stalling when coming to a stop
- Stuck throttle at a fixed RPM
What causes the P2138 code?
Potential causes of P2138 include:
- Faulty throttle position sensor
- Defective accelerator pedal position sensor
- Damaged throttle body
- Wiring issues in the APP or TP circuits
- Corroded electrical connectors
- Faulty ECM or outdated ECM software
Is it safe to drive with a P2138 code?
No, it is not recommended to drive a vehicle with an active P2138 code. This fault can cause a sudden loss of throttle control, unintended acceleration, or an inability to change engine speed, which poses a serious safety risk. Have the vehicle diagnosed and repaired as soon as possible.
How do you diagnose the P2138 code?
To diagnose P2138, a technician will typically:
- Visually inspect the throttle body, APP sensor, and wiring
- Test the TP and APP sensor reference voltages and grounds
- Observe sensor data with a scan tool while operating the accelerator
- Perform a throttle body cleaning service
- Check for damaged wires or poor connections in the sensor circuits
- Test the ECM for proper operation
Can a dirty throttle body cause the P2138 code?
Yes, a dirty or sticking throttle body can cause a correlation error between the APP and TP sensors, triggering code P2138. Cleaning the throttle body and idle air control valve may resolve the issue in some cases.
How much does it cost to fix the P2138 code?
The cost to repair a P2138 fault varies depending on the root cause. Throttle position sensor replacement typically costs between $120-$300 for parts and $80-$120 for labor. Accelerator pedal sensor replacement and throttle body cleaning are similarly priced. Wiring repairs can range from $50 to over $500. In rare cases, a faulty ECM may need replacement at a cost of $800-$1500.
Can I diagnose the P2138 code myself?
Diagnosing P2138 often requires advanced knowledge of throttle control systems, specialized diagnostic equipment, and access to vehicle-specific repair information. While an experienced DIYer may be able to diagnose the issue with the proper tools and resources, it is generally recommended to have the vehicle professionally diagnosed to avoid further damage.
What is the difference between the APP sensor and TP sensor?
The accelerator pedal position (APP) sensor measures the position of the accelerator pedal and sends a corresponding voltage signal to the ECM. The throttle position (TP) sensor is mounted on the throttle body and measures the actual position of the throttle plate. The ECM compares the APP and TP sensor signals to determine the proper throttle opening.
Can a faulty accelerator pedal cause the P2138 code?
Yes, a defective accelerator pedal position sensor or damaged wiring in the accelerator pedal circuit can cause a voltage discrepancy with the throttle position sensor, triggering code P2138.
What does "voltage correlation" mean in the P2138 code description?
"Voltage correlation" refers to the relationship between the voltage signals from the accelerator pedal position sensors and throttle position sensors. The ECM expects these voltages to match within a specified range. If the voltages differ significantly, it indicates a problem with one of the sensors or their circuits.
Can a weak battery cause the P2138 code?
While a weak battery itself is unlikely to cause code P2138, low system voltage can affect the operation of the electronic throttle control system. Ensure the vehicle's charging system is functioning properly and the battery is fully charged before diagnosing throttle-related faults.
What is "limp mode"?
Limp mode, also known as "fail-safe mode," is a safety feature that limits engine performance when a critical fault is detected. In the case of code P2138, the ECM may limit throttle opening to a fixed percentage, such as 20-30%, to prevent unintended acceleration. Limp mode allows the vehicle to be driven at a reduced speed to a repair facility.
Can a P2138 code cause transmission problems?
Code P2138 is primarily an engine control issue, but it can indirectly affect transmission operation. The ECM may limit engine power in response to a throttle correlation fault, which can cause poor acceleration, rough shifting, or a lack of responsiveness from the transmission.
What other codes may be related to P2138?
Other diagnostic trouble codes that may appear along with P2138 include:
- P2135 – Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation
- P2122 – Throttle/Pedal Position Sensor/Switch D Circuit Low Input
- P2127 – Throttle/Pedal Position Sensor/Switch E Circuit Low Input
- P2101 – Throttle Actuator Control Motor Circuit Range/Performance
- P0121 – Throttle/Pedal Position Sensor/Switch C Circuit Range/Performance
Can a software update fix the P2138 code?
In some cases, an outdated or faulty ECM calibration can cause throttle control issues. Reflashing the ECM with the latest software update may resolve code P2138 if no hardware faults are found. However, always diagnose the system thoroughly before assuming a software issue.
How often should I clean the throttle body to prevent the P2138 code?
Throttle body cleaning is typically recommended every 30,000 miles or as specified in the vehicle's maintenance schedule. Carbon buildup on the throttle plate and bore can cause sticking and erratic operation, leading to correlation faults. Regular cleaning with a quality throttle body cleaner can help prevent these issues.
Can a P2138 code damage the catalytic converter?
Code P2138 itself does not directly damage the catalytic converter. However, if the underlying cause of the code is not repaired promptly, it can lead to incomplete combustion and an overly rich fuel mixture. Over time, this can cause the catalytic converter to become fouled or overheated, reducing its efficiency and lifespan.
What should I do if the P2138 code keeps coming back after repair?
If the P2138 code persists after replacing sensors or cleaning the throttle body, there may be an underlying wiring issue or a faulty ECM. Double-check all wiring and connectors for damage or poor connections. If no wiring faults are found, the ECM may need to be reprogrammed or replaced. Consult with a professional technician for further diagnosis.
How long does it take to diagnose and repair the P2138 code?
The time required to diagnose and repair code P2138 varies depending on the root cause and the specific vehicle. A simple throttle body cleaning or sensor replacement may take 1-2 hours, while wiring repairs or ECM replacement can take several hours. Expect a thorough diagnosis to take at least 1 hour, plus additional time for repairs.
By understanding the causes, symptoms, and diagnostic steps for code P2138, you can make an informed decision about how to address this issue on your vehicle. When in doubt, consult with a qualified technician for an accurate diagnosis and repair. Proper maintenance, including regular throttle body cleaning, can help prevent this code from occurring.