Error Code P0638 is defined as Throttle Actuator Control Range/Performance (Bank 1). This code is a generic trouble code, which means it applies to all vehicles equipped with OBD-II system, especially those made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

The monitor whether the position of the actual throttle blade is in or outside of range, the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) uses the TPS (throttle position sensor). If it sees it’s out of range, then it registers the Error Code P0638 and activates the Check Engine light. Bank 1 on the description refers to the side of the engine that contains the cylinder number 1. Most vehicles, however, use throttle body for all cylinders.

Most throttles of this type are NOT serviceable, therefore must be replaced. The throttle body is spring actuated that holds an open position in the event of a motor failure, in some cases, with complete failure, the throttle will be unresponsive, rendering the vehicle to move only at slow speed.

Other related trouble code:

If both Error Code P0638 and Error Code P0639 are present, then you can suspect a fault in the wiring, lack of voltage, or PCM/ECM problem.

Common Symptoms

  • Hesitation upon acceleration, especially when you put the pedal down

Possible Causes

There are a number of potential causes that lead to this code. Some of the common causes include:

  • Defective pedal position sensor
  • Defective throttle position sensor
  • Defective throttle actuator motor
  • Dirty throttle body
  • Dirty or loose connection
  • Dirty or loose wiring harness

How to Check

First Step

As with most fault codes, the first thing to do is to record all fault codes present, including all freeze frame data. This information can be used should intermittent condition is proven later on diagnosis.

Note: If this code is shown as “pending code,” that means more fault code cycles must occur before this code becomes “active” and activate the Check Engine light. However, this code still must not be ignored, since there are already many “active” CAN bus related codes present.

Second Step

Next, refer to the manual to locate the throttle actuator and determine the location, function, color-coding, and routing of all associated wirings.

Inspect all the connectors and wires thoroughly. Look for signs of damages on the connectors and wires, such as disconnected, corroded, burnt, damaged, etc. Conduct all necessary repairs or replacements on the connectors and wires. Reset and then take the vehicle for a test drive before scanning the system again to see if the code comes back.

Third Step

If the code comes back, check for the performance reference voltage, ground connectivity, continuity, and resistance of the wirings. Prevent controller damage by making sure to disconnect the throttle actuator wiring from the PCM.

Follow the directions in the manual thoroughly to prevent the possibility of obtaining wrong or misleading results, especially on the signal circuit of the throttle actuator position switch.

Compare all the results with the values you see from the manual. If you find any discrepancies, this means you have to replace the wirings to ensure all electrical values fall within the specifications.

Note: Make sure to test the stepper motor and position switch from part of the control circuit. Check with the manual for detailed information on the right procedure(s) to follow to test both components. Internal resistance for these components is a good indicator for serviceability (or otherwise). Thus, any components with internal resistance not matching manufacturer’s specifications must be replaced.

Also, since this code involves the throttle control, “close enough” is definitely NOT good enough. All electrical values must match exactly, or very nearly. Sloppy diagnosis can result to complete loss of throttle control, which could lead to more serious problems.

Clear the codes after this step, and operate the vehicle for one drive cycle before scanning again to see if the code comes back.

Fourth Step

If all electrical values are within the manufacturer’s specified ranges and the code persists, then you can suspect a defective throttle actuator or presence of carbon deposits that could prevent or inhibit free movement of the throttle plate.

Check the throttle body by removing it from the inlet tract. Look for signs of damage and carbon deposits. If there are no damages, then remove carbon buildup using approved solvent. You may need more than one application of a solvent to clean it completely.

If the scanner has control functions, check for signs of intermittent faults by reconnecting the wiring on the throttle body and use the scanner to command the actuator to rotate the throttle plate from fully closed, to fully open position multiple times.

Take note of the values displayed on the scanner in this step. Some scanners show display position of the throttle plate in the degrees of rotation, while others will show a percentage of the opening. Regardless of this, both values for fully closed and fully open must be the same or at least very close (within same percentage point/degree) to the values specified by the manufacturer. If the values don’t agree, replace the position sensor and then repeat this step.

Also, replace the throttle body/actuator assembly as a complete unit if the mechanism doesn’t move freely, or if there are signs of sticking, binding, or erratic movement of the throttle plate.

Note: In some cases, throttle body must be grounded to the engine for this step to work. Also, on many applications, the idle relearning procedure must be completed when the throttle actuator or position switch is replaced. Thus, consult with the manual for the detailed information on both testing and relearning process.

If the scanner doesn’t have control functions, check the manual on possible workaround methods to test the throttle actuator’s operation. If the manual doesn’t openly describe a method of applying direct current to the throttle actuator for the purpose of testing the unit, do NOT apply direct current to any of the terminal or pin in the connector, as this could cause extensive damage to the electrical system and more control modules.

Fifth Step

Put the throttle body back, reconnect the wirings, and then clear the codes. Then, connect the scanner and take the vehicle for at least one drive cycle. Monitor the operation of the throttle control system, and operation of the throttle position sensor in particular.

If the code doesn’t come back, don’t assume that the pair had been successful. Consult with your manual on the number of fault cycles requires to set the code. Operate the vehicle for the number of drive cycles before scanning again to see if the code returns. If the code doesn’t return after many completed drive cycles, then you can assume a successful repair.

If the code comes back immediately after a couple of drive cycles, then you can assume an intermittent fault, or the fault may be buried in the CAN bus system. In both cases, the best action is to take the vehicle to the dealer, or to a competent repair shop for a professional diagnosis.

How to Fix

Depending on the diagnosis, common repairs for this code are:

  • Cleaning the throttle body; removing carbon buildup in throttle body using approved solvent
  • Replacement of faulty throttle actuator motor
  • Aside from throttle actuator motor, mechanics may have to replace the entire throttle body, including the throttle position sensor
  • In some cases, manufacturers will also recommend replacement of the pedal sensor when you replace the throttle body
  • Replacement of bad connections (loose, corroded, or damaged wirings and connectors)
  • Fixing of faulty control modules
  • Replacement of faulty wiring within the CAN Bus harness

If there are other trouble codes associated with the throttle position sensor, it’s best to address them first before diagnosing P0638.

Many of the possible causes of this code a fairly simple. A lot of times, it can be fixed by simply cleaning those dirty components. However, if you go through the diagnosis and none of them seems to be causing the problem, then you can suspect a malfunctioning PCM.