Comprehensive diagnostic guide for OBD-II code P0638

Throttle Actuator Control Range/Performance (Bank 1)

Code and definition

• P0638 is a Powertrain/OBD-II code indicating a problem with the Throttle Actuator Control (TAC) system, specifically its range or performance for Bank 1. In other words, the TAC actuator (and/or its feedback sensors) is not moving or reporting within expected limits compared to what the engine control module (ECM/PCM) commands. This definition aligns with the open-source code listing describing and standard TAC terminology used in OBD-II references. Also, OBD-II diagnostic trouble code pages describe DTCs generally as monitors that detect issues in modern electronic engine controls and the TAC is a common target for throttle-control related codes.

Symptoms

• Poor throttle response: delayed or no response to pedal input; engine hangs at an unintended RPM or speed.
• Idle quality issues: rough idle, hunting, stalling, or fluctuating idle speed.
• Limp or degraded performance: reduced power, limited RPM, or "limp-home" mode to protect the engine.
• Check Engine Light (CEL) illumination with P0638 stored or pending in the PCM.
• Intermittent drivability, especially when changing load quickly (e.g., accelerating onto a highway on-ramp).
  Note: These symptom clusters are consistent with TAC range/performance concerns described by TAC/OBD-II concepts.

Probable Causes

• TAC motor or throttle body failure: worn, sticking, or non-responsive throttle actuator that cannot meet commanded positions.
• Wiring harness or connector problems to TAC motor or throttle position sensor (TPS): corroded pins, damaged insulation, loose connectors, or compromised grounds/power supply.
• Throttle body contamination or mechanical binding: carbon/deposits or mechanical binding inside the bore, linkage, or plate causing out-of-range movement.
• Throttle Position Sensor (TPS) or feedback sensor fault: incorrect position reporting, miscalibrated or intermittent TPS readings that mismatch commanded TAC position.
• PCM/ECU calibration or software issue: faulty or outdated software firmware that misinterprets TAC feedback or mis-commands the TAC.
• Vacuum leaks or intake system issues that indirectly affect TAC readings or throttle calibration.

Notes on causes and their likelihood:

• The TAC motor/wiring and throttle body assembly problems are typically the most common root causes in many vehicles.
• TPS faults and wiring issues are also frequent contributors.
• ECU software issues are less common but can produce P0638 in some vehicles.
• Vacuum leaks and intake system issues are possible contributors, especially if they affect throttle measurements or sought-after valve behavior.

Diagnostic Approach

1) Confirm and gather data

• Confirm that P0638 is the active code and note any related codes (e.g., P0120, P022 controlling sensors, etc.). Retrieve freeze-frame data to understand engine speed, requested vs actual throttle positions, and any abnormal sensor readings at the time the code set.
• Use live data to compare: commanded throttle angle vs actual throttle angle, TAC motor duty cycle, and TPS voltage. Look for large discrepancies or slow/erratic TAC response.

2) Visual and basic electrical inspection

• Inspect the throttle body and TAC assembly for obvious signs of damage, sticking, dirt, or carbon buildup.
• Inspect wiring harnesses and connectors to the TAC motor and throttle position sensor: pin integrity, corrosion, bent pins, frayed wires, and secure connector latches.
• Check power supply and ground paths to the TAC circuit and TPS circuit. A poor ground or Vref supply can produce incorrect readings.

3) Inspect the intake system

• Look for vacuum leaks near the intake manifold, vacuum hoses, or brake booster lines that could affect readings and overall throttle calibration.

4) Evaluate the TAC system function (if testable with the vehicle's scan tool)

• If the vehicle supports an actuator test, command the TAC to 0% and 100% (and intermediate steps) and observe whether the throttle plate moves correspondingly and smoothly without binding or stalling.
• Compare commanded position to actual position during the test. A large or inconsistent mismatch indicates a TAC or feedback issue.

5) Inspect the TPS and related sensors

• Verify TPS output is smooth and within expected voltage range as the throttle moves. Look for abrupt changes or dead spots.
• If the TPS voltage is erratic or out of spec, the TPS may be the root cause or a contributor to P0638.

6) Electrical diagnostics

• Perform continuity and resistance checks on TAC motor circuit wires and TPS signal and power circuits.
• Check for proper Vref (5 V typically, or a vehicle-specific reference) and ground integrity.

7) Software/Calibration considerations

• Check for any available ECU software updates or recalibration procedures for TAC system. Some vehicles require a re-learn or calibration after components are replaced to restore proper TAC range/response.
• If a software issue is suspected, consult the vehicle manufacturer's diagnostic/flash tool procedures.

8) Diagnostic decision point

• If TAC motor, wiring, and connectors test OK but the issue persists, consider replacing the TAC/throttle body assembly or TPS, followed by re-learning/calibration as required.
• If the fault is intermittent and tied to a particular operating condition (e.g., hot engine, specific RPM), gather more data to narrow down whether the problem is mechanical, electrical, or software related.

Tests and measurements to perform (practical examples)

• Live data: observe commanded throttle vs actual throttle position and TAC motor duty cycle across the full pedal travel.
• TPS voltage sweep: verify the TPS output changes smoothly with pedal movement; look for dead zones or abrupt jumps.
• Resistance checks: verify continuity of TAC motor supply and ground lines; verify proper resistance values for sensors if specified by the vehicle's service information.
• Actuator test: safe, controlled TAC movement from low to high positions; confirm no mechanical interference.
• Visual inspection: throttle plate motion free of binding; no sticking or unusual resistance.

Common repair strategies (in order of likelihood)

• Clean or replace throttle body/tac assembly if sticking or carbon buildup is present.
• Repair/replace damaged wiring or connectors to TAC and TPS; fix any grounds or Vref supply issues.
• Replace TPS or TAC sensor components if sensor faults are confirmed.
• Relearn or reprogram ECU software or perform software update if indicated by the OEM procedure.
• Replace the throttle body/tac assembly if the actuator itself is failing (non-responsive or significantly out of range) after all other simpler fixes are exhausted.
• After repairs, perform a thorough test drive and re-check for DTCs to ensure the fault does not recur.

Post-Repair Verification

• Clear codes and perform a road test to ensure no return of P0638.
• Recheck live data for stable throttle response across the pedal range; verify there is no mismatch between commanded and actual positions.
• Confirm the absence of related codes such as TPS or sensor-related DTCs.
• Confirm that idle quality and engine response are normal and that there are no other drivetrain fault codes present.

Safety Considerations

• Electric throttle systems involve high-current controls; follow manufacturer safety procedures when performing electrical tests.
• Disconnecting the battery or performing certain tests can reset PCM data or misalign learned throttle positions; perform re-learn/calibration as required.

Cause Probability

• TAC motor/wiring/connector faults: ~40%

• Throttle body contamination or mechanical binding: ~20%

• TPS sensor or TPS circuit faults: ~15%

• PCM/ECU software/calibration issues: ~10%

• Vacuum/intake leaks affecting readings: ~10%

• Other miscellaneous causes (less common): ~5%

• Diagnosis and general understanding of DTCs and powertrain codes are described in Wikipedia: OBD-II > Diagnostic Trouble Codes and OBD-II > Powertrain Codes (context for how DTCs monitor parameters and generate codes as part of engine control monitoring).

• The specific code P0638 corresponds to Throttle Actuator Control Performance/Range (Bank 1) per the Open Source repository listing (MIT), which provides the standard description used here.

• If conflicts arise between general DTC concepts and the specific code definition, the code's TAC-range/performance interpretation takes precedence for P0638, with the general DTC framework providing context for how the TAC system operates and is monitored.

What you should do next (summary)

• Start with a careful visual and electrical inspection of the TAC/throttle body, TPS wiring, and connectors.
• Check for vacuum leaks and intake system issues that could influence throttle behavior.
• Use live data to compare commanded vs actual TAC position and TPS readings; perform TAC actuator tests if your scan tool supports them.
• Address wiring/connectors, clean the throttle body if needed, replace failed sensors or actuator components as indicated by your findings.
• Update or re-learn ECU calibration if software updates are available or after component replacement.
• Verify the repair with road testing and confirm that P0638 does not reappear.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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