Fuel and air

P0638 — Throttle Actuator Control Range/Performance

Severity: medium Drive with care, to a shop

What P0638 means

The engine control module calculates the throttle angle it needs and drives an electric motor to reach it. Position sensors report the actual angle back. When the actual angle differs too much from the command, the module sets P0638 and usually limits engine power.

Symptoms

  • Check Engine light, sometimes a throttle warning light
  • Reduced engine power or limp mode
  • Surging or unstable idle
  • Poor or jerky response to the gas pedal
  • Stalling at stops or hard starting

Possible causes

  1. Dirty throttle body with carbon buildup on the plate and bore
  2. Worn or failed throttle actuator motor
  3. Throttle relearn not done after cleaning or replacement
  4. Corroded, open or shorted wiring or connector at the throttle body
  5. Faulty accelerator pedal position (APP) sensor
  6. Mechanical binding of the throttle plate or an intake air leak
  7. Low system voltage or a poor ECM ground

How to diagnose

  1. Scan all codes and freeze frame data, and look for related throttle or pedal codes
  2. Inspect the throttle body connector and harness for corrosion and damage
  3. Remove the intake duct, check the plate for carbon and sticking, and move it by hand with the ignition off
  4. With a scan tool, compare commanded and actual throttle angle while slowly pressing the pedal
  5. Check APP sensor readings for smooth signals without dropouts
  6. Test motor circuit voltage and resistance, plus grounds and the 5V reference
  7. Check battery voltage and charging output
  8. After cleaning or replacement, run the throttle relearn and road test

Common fixes

  • Clean the throttle body with throttle-safe cleaner
  • Perform the throttle relearn with a scan tool
  • Repair the wiring or connector
  • Replace the throttle body assembly
  • Replace the accelerator pedal sensor

P0638 on different brands

GM
On GM vehicles this code often appears together with P1516, which points to a mismatch between commanded and actual throttle position.

Questions and answers

Can I drive with P0638?

Only briefly and carefully, such as to a repair shop. Power is usually limited and the throttle can respond late, so avoid situations like passing.

Will cleaning the throttle body fix P0638?

Often, if carbon is the cause. Always do a throttle relearn afterward, or the code and rough idle may return.

Do I need a new throttle body for P0638?

Not always. Check for dirt, wiring faults, the pedal sensor and the relearn first. Replace it if the motor is worn or the plate doesn't follow commands.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.