P0638

What it means

P0638 means the engine control unit has detected that the electronic throttle actuator is not moving the throttle plate to the position it commanded, or is not doing so within the expected range or time. The unit compares the requested throttle angle with the angle reported by the throttle position sensors and sets the code when the difference is too large or the response is too slow. Most vehicles will limit engine power in response.

Symptoms

• Engine management light on, often with a reduced power or limp mode warning
• Poor or delayed throttle response
• Engine speed stuck at a fixed fast idle or limited to a low level
• Rough, unstable or low idle, or stalling when stopping
• Hesitation or surging when pulling away
• Cruise control not working

Possible causes

• Carbon or dirt build-up in the throttle body restricting or sticking the throttle platelikely
• Worn or failing throttle actuator motor or gears inside the throttle bodylikely
• Damaged wiring, corroded terminals or loose connector in the throttle actuator or position sensor circuitpossible
• Faulty throttle position sensor or accelerator pedal position sensor giving implausible readingspossible
• Low or unstable supply voltage or a poor earth, caused by a weak battery or charging faultpossible
• Air leak or foreign object affecting throttle plate movement or the learned throttle positionrare
• Fault in the engine control unit driver circuit or outdated software, where a known issue exists for the modelrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting related throttle, pedal position and voltage codes
2. Check battery condition and charging voltage, as low voltage can upset the throttle actuator
3. Visually inspect the throttle body wiring and connector for damage, corrosion, chafing, water ingress or poor pin tension
4. Inspect the throttle plate for carbon build-up, sticking or debris, and check for free movement by hand with the ignition off where the design allows
5. Using a scan tool, compare commanded and actual throttle angle and the sensor signals while the throttle is operated, looking for lag, jumps or dropouts
6. Check the accelerator pedal sensor signals for smooth, plausible readings across the full pedal travel
7. Test the actuator circuit supply, earth and signal wiring for continuity and voltage drop, following the manufacturer's wiring diagram
8. Check for any applicable software updates or technical bulletins for the specific model and engine

How to fix

• Clean the throttle body using a suitable cleaner, following the manufacturer's procedure, then carry out a throttle adaptation or relearn if the vehicle requires it
• Repair or replace damaged wiring, terminals or connectors in the throttle circuit
• Replace the throttle body or actuator assembly if the motor, gears or sensors are worn or faulty, then perform the adaptation procedure
• Replace a faulty accelerator pedal sensor if it is found to be giving implausible signals
• Replace a weak battery or repair the charging system or earth connections
• Update the engine control unit software only where an outdated calibration is a known cause for that model

Can I keep driving?

Nearby codes

• P0637GenericPower Steering Control Circuit High
• P0639GenericThrottle Actuator Control Range/Performance Bank 2