The OBD2 code P0638 indicates an issue with the “Throttle Actuator Control Range/Performance,” meaning the powertrain control module (PCM) has detected a discrepancy in the performance or position of the throttle actuator or motor. This can lead to erratic throttle response, poor engine performance, and in some cases, a lack of throttle response. This code is particularly challenging because it involves both electronic and mechanical components of the throttle control system.
P0638 quick overview
| Meaning | P0638: Throttle Actuator Control Range/Performance |
| Is it serious? | Yes, it can significantly affect vehicle drivability and safety. |
| Possible causes | – Faulty throttle body or actuator – Incorrect installation or calibration of the throttle body – Wiring issues or poor connections at the PCM or throttle body – Incompatibility issues with aftermarket components |
| How to diagnose? | Inspect the throttle body for cleanliness, damage, or improper installation. Check wiring and connectors for damage, corrosion, or loose connections. Test the throttle actuator for proper operation using specific diagnostic tools. Consider ECM/PCM issues, including programming errors or compatibility with immobilizer systems. Evaluate aftermarket parts for compatibility and performance issues. |
P0638 Meaning
When your car shows the P0638 code, it’s like saying there’s a miscommunication or problem with the part that controls how fast or slow your engine goes (the throttle). This could mean the throttle isn’t opening or closing as it should, which might make your car act up, like not speeding up properly or idling roughly.
The problem could be with the throttle mechanism itself, the wires connecting it to the car’s brain (the PCM), or even just because a part isn’t fitting correctly, especially if it’s not a part originally made for the car.
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How to fix P0638 – learn from mechanics
I looked into P0638 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.
Case #1: Subaru
Vehicle Information 2005 Subaru Outback XT Limited 2.5L
Issue + Repair History
- Experiencing a random OBD2 trouble code P0638 (Throttle Actuator Control Performance).
- Initial diagnostics suggested ECM replacement.
- Consideration of using a used ECM raised questions regarding compatibility with immobilizer system.
Other OBD2 Codes + Test Results
- Only the P0638 trouble code mentioned.
Mechanic Suggestions
- Perform keys registration using a Subaru scan tool after ECM replacement due to the vehicle’s immobilizer system.
- Consider trying a used throttle body as an alternative before replacing the ECM.
- Ensure thorough cleaning of the throttle body as a potential troubleshooting step.
Final Fix After attempting to replace the ECM and facing compatibility issues with the immobilizer system, the technician opted to return the used ECM. Instead, a used throttle body was installed, which resolved the issue without triggering any further trouble codes. This experience emphasized the importance of understanding manufacturer specifications and considering alternative solutions.
Case #2: Mazda
Vehicle Information 2008 Mazda 3 S 2.3L
Issue + Repair History
- Experiencing idle faults with no throttle response and MIL light on.
- OBD2 trouble codes P0638 (Throttle Control Range/Performance Fault) and P2112 (TAC Performance Problem) present.
- Engine shakes badly in gear with cylinders #2 & 3 coils not firing.
- Previous repair attempts included throttle body replacement by the technician and subsequent replacement by the dealer along with the PCM.
Other OBD2 Codes + Test Results
- Only the P0638 and P2112 trouble codes mentioned.
Mechanic Suggestions
- Consider checking the circuit with the fuel pump fuse, as pulling this fuse for injector cleaning caused similar symptoms in another case.
- Ensure thorough inspection and testing of wiring connections, especially the TAC control wire at the ECM, as loose pin tension can cause similar symptoms.
Final Fix Upon closer inspection, a small black burn mark was found on the PCM connector for the white/green wire. The terminal appeared to be spread open, causing a lack of command from the PCM to open the throttle plate. After tightening the terminal and reconnecting the PCM, the issue was resolved, demonstrating the importance of meticulous inspection and addressing basic wiring issues.
Case #3: Kia
Vehicle Information 2016 Kia Forte EX 2.0L
Issue + Repair History
- Experiencing persistent P0638 Throttle Actuator Range Performance code despite throttle body replacement with an aftermarket unit.
- Inconsistent throttle response, with instances of no response at all.
- Throttle body replaced by the used car lot and subsequently replaced again by the dealer along with the PCM.
- Throttle body connector and wiring inspected, showing no apparent issues.
Other OBD2 Codes + Test Results
- Only the P0638 trouble code mentioned.
Mechanic Suggestions
- Consider the possibility of the aftermarket throttle body being out of normal range, leading to improper adaptation by the PCM.
- Ensure proper installation of OE throttle body unit and verify connector pin placement to prevent potential wiring issues.
- Investigate for any signs of tampering with the connector or forced back probing, which could affect signal transmission.
- Monitor PCM signals from the throttle body wires to identify potential signal discrepancies or interference.
- Highlight the importance of using a scanner capable of clearing adaptive values from the PCM, especially if previous issues with the throttle body were present.
- Pay close attention to the throttle body connector for loose terminal pins, as this could affect signal integrity and performance.
Final Fix The VIN check revealed a salvage title, indicating a potential history of damage or previous issues with the vehicle. Combined with insights from a master Kia tech, the recommendation was made to involve the dealer for thorough diagnostics and potential resolution. This decision aimed to address concerns about the aftermarket throttle body and ensure proper handling of any underlying issues.
How to diagnose P0638
I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.
| Diagnostic Step | Brief Explanation |
|---|---|
| Inspect throttle body and actuator | Check for physical damage, cleanliness, and proper install. |
| Check wiring and connectors | Look for damage, corrosion, or looseness affecting signals. |
| Test throttle actuator function | Use diagnostic tools to ensure correct operation. |
| Evaluate ECM/PCM and immobilizer compatibility | Ensure replacement parts are compatible with vehicle systems. |
| Assess aftermarket part installation | Verify compatibility and performance of non-OE components. |
The service cases from a Subaru Outback, Mazda 3, and Kia Forte illustrate the complexity of diagnosing and resolving issues indicated by the P0638 code.
These examples highlight the importance of considering both the mechanical and electronic aspects of the throttle control system, the potential impact of aftermarket parts, and the need for precise diagnostics and compatibility checks, especially when dealing with critical components like the throttle body and ECM/PCM.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko