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P0638Throttle Actuator Control Range/Performance (Bank 1)

The electronic throttle actuator on Bank 1 is not moving the throttle plate to the position the PCM is commanding.

Definition

On drive-by-wire engines, the PCM commands the throttle actuator motor and verifies the result through the throttle position sensors. P0638 sets when the actual throttle angle deviates from the commanded angle beyond the allowed tolerance for a calibrated time, indicating the actuator is binding, slow, or not responding correctly. The system typically responds by forcing reduced engine power (limp mode) with a fixed or limited throttle opening, illuminating the MIL and often a wrench or ETC lamp, and the vehicle may exhibit poor acceleration, fluctuating idle, or stalling. Because throttle control directly affects driveability and safe merging/passing power, this code should be addressed promptly.

Common causes

  • Carbon buildup on the throttle plate and bore causing the actuator to bind or stick
  • Worn throttle actuator motor or stripped internal gears in the throttle body
  • Wiring or connector faults in the throttle actuator motor circuits
  • Failed throttle position sensor feedback skewing the actual-position reading
  • Low system voltage limiting actuator motor torque
  • PCM throttle driver fault or a lost throttle relearn after cleaning or battery disconnect

Common misdiagnoses

  • Replacing the accelerator pedal position sensor, which has its own dedicated codes
  • Replacing the throttle body when cleaning and a relearn procedure would have fixed it
  • Replacing the PCM before checking actuator wiring and supply voltage
  • Replacing the MAF sensor for what is a throttle plate control fault

Troubleshooting steps

  1. 1.Compare commanded versus actual throttle position on a scan tool

    Watch the desired and actual throttle angle PIDs during a key-on sweep and snap throttle; deviation greater than a few percent or a lagging response confirms the range/performance fault.

  2. 2.Inspect and clean the throttle body

    Remove the intake duct and check for heavy carbon at the plate edges and bore; clean with throttle body cleaner without forcing the plate by hand on torque-sensitive designs.

  3. 3.Check the actuator motor circuits

    Inspect the throttle body connector for bent or corroded pins and verify the motor winding resistance is within specification, typically a few ohms.

  4. 4.Verify battery and charging voltage

    Confirm cranking voltage stays above about 9.6 volts and running voltage is 13.5-15.0 volts, since low voltage weakens actuator response and can set this code.

  5. 5.Perform the throttle relearn procedure

    After any cleaning, battery disconnect, or part replacement, complete the manufacturer's idle/throttle relearn with a scan tool or the specified key-cycle sequence.

  6. 6.Retest under load

    Clear codes and drive through several full accelerator applications, confirming actual throttle tracks commanded angle without triggering reduced-power mode.

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