P0638
Throttle Actuator Control Range/Performance (Bank 1)
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The P0638 code indicates that the PCM has detected the electronic throttle on Bank 1 is not performing within its expected range. In modern drive-by-wire vehicles, there is no physical cable connecting the gas pedal to the throttle. Instead, the accelerator pedal position sensor tells the PCM how much throttle you want, and the PCM commands an electric motor in the throttle body to open the throttle plate to the correct angle. When the actual throttle position doesn't match the commanded position, this code is set.
This code triggers an immediate limp mode on most vehicles, which dramatically reduces available power and limits your speed — typically to around 20-25 mph. This is a safety measure designed to give you enough power to pull over or limp home, but not enough to create a dangerous situation with an unreliable throttle. The engine may idle roughly, stall, or feel completely unresponsive to the gas pedal.
The first thing to try is cleaning the throttle body, which is a common and inexpensive fix. Remove the intake tube from the throttle body and spray the interior with dedicated throttle body cleaner, wiping away carbon deposits on the plate and bore. After cleaning or any throttle body work, most vehicles require a throttle body relearn procedure (often done by turning the key on, waiting, cycling the key, etc. — check your vehicle's specific procedure). If cleaning doesn't help, the throttle body assembly itself may need to be replaced. Also inspect the wiring and connectors to the throttle body and accelerator pedal position sensor for damage.
Severity
Symptoms
- •Vehicle enters limp mode with very limited acceleration (typically under 25 mph)
- •Check engine light illuminated
- •Engine idles roughly or stalls
- •Accelerator pedal feels unresponsive or has delayed response
- •Engine RPMs do not match throttle pedal position
- •Reduced power warning message on the dashboard (if equipped)
Likely Causes
The throttle body contains an electric motor that opens and closes the throttle plate. When this motor wears out, sticks, or fails, the actual throttle position won't match the commanded position, causing a range/performance error.
Over time, carbon deposits, oil vapor, and grime accumulate on the throttle plate and bore. This buildup restricts movement and prevents the throttle from reaching its commanded positions accurately.
The TPS reports the actual throttle blade angle to the PCM. If the sensor is worn or sending inaccurate readings, the PCM perceives a mismatch between commanded and actual throttle position even if the throttle is moving correctly.
Damaged, corroded, or loose wiring between the PCM, accelerator pedal position sensor, and throttle body can cause signal drops or interference that result in out-of-range performance readings.
If the pedal position sensor sends erratic or incorrect signals, the PCM will command incorrect throttle positions, creating a mismatch between expected and actual throttle performance.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Clean the throttle body with throttle body cleaner to remove carbon deposits
- Replace the electronic throttle body assembly if the motor or internal components have failed
- Replace the throttle position sensor if it is reading inaccurately
- Inspect and repair wiring and connectors in the electronic throttle control circuit
- Perform a throttle body relearn procedure after cleaning or replacement