What does it mean?
Some newer vehicles are equipped with drive-by-wire systems – where the throttle body is controlled by a sensor on the accelerator pedal, the powertrain control module / engine control module (PCM/ECM), and an electric actuator motor in the throttle body.
The PCM/ECM uses the throttle position sensor (TPS) to monitor the actual position of the throttle blade, and when the actual position is out of range compared to the target position, the PCM/ECM sets the DTC P0638. Row 1 refers to the side of the engine with cylinder number one, but most vehicles use a single throttle body for all cylinders. This code is similar to code P0639.
Most throttle bodies of this type are not repairable and must be replaced. The throttle body is spring-actuated to maintain an open position in case of motor failure; in some cases with a complete failure, the throttle will not respond and the vehicle will only be able to drive at a slow speed.
Note: If there are DTC codes associated with the throttle position sensor, be sure to correct them before diagnosing code P0638.
Symptoms
Symptoms of a P0638 trouble code may include:
- The engine light is on (malfunction indicator lamp)
- The vehicle may feel like it hesitates during acceleration
Potential causes of this DTC may include:
- Malfunctioning pedal position sensor
- Malfunctioning throttle position sensor
- Malfunctioning throttle actuator motor
- Dirty throttle body
- Wiring harness, loose or dirty connections
- PCM/ECM malfunction
Diagnostic/Repair Steps
Pedal Position Sensor – The pedal position sensor is located on the accelerator pedal. There are typically three wires used to determine the pedal position: a 5-volt reference supplied by the PCM/ECM, ground, and the sensor signal. A factory wiring diagram will be needed to determine which wire is used. Ensure the connection is secure and there are no loose wires in the harness. Check for proper grounding using a digital volt ohmmeter (DVOM) set to the ohms scale by connecting one wire to ground at the sensor connector and the other to chassis ground – the resistance should be very low. Check the 5-volt reference voltage from the PCM using the DVOM set to a volts scale with the positive lead at the harness connector and the negative lead on a known good ground with the key in the on or run position.
Check the reference voltage using the DVOM set to a volts scale with the red lead on the reference signal and the negative lead on a known good ground with the key in the on/run position – the signal voltage should increase as the accelerator pedal is pressed further. Typically, the voltage ranges from 0.5 volts with the pedal not pressed to 4.5 volts at wide-open throttle position. It may be necessary to also check the signal voltage at the PCM to determine if there is a voltage difference between the sensor and what the PCM reads. The position sensor signal should also be checked with a graphing multimeter or oscilloscope to determine if the voltage increases smoothly without interruption throughout the entire range of motion. If an advanced scan tool is available, the position sensor is usually displayed as a percentage of the desired throttle; ensure the desired value is similar to the actual pedal position.
Throttle Position Sensor – The throttle position sensor monitors the actual position of the throttle body blade. The throttle position sensor is located on the throttle body. There are typically three wires used to determine the pedal position: a 5-volt reference supplied by the PCM/ECM, ground, and the sensor signal. A factory wiring diagram will be needed to determine which wire is used. Ensure the connection is secure and there are no loose wires in the harness. Check for proper grounding using a digital volt ohmmeter (DVOM) set to the ohms scale by connecting one wire to ground at the sensor connector and the other to chassis ground – the resistance should be very low. Check the 5-volt reference voltage from the PCM using the DVOM set to a volts scale with the positive lead at the harness connector and the negative lead on a known good ground with the key in the on or run position.
Check the reference voltage using the DVOM set to the volts scale with the red lead on the reference signal and the negative lead on a known good ground with the key in the on/run position – the signal voltage should increase as the accelerator pedal is pressed further. Typically, the voltage ranges from 0.5 volts with the pedal not pressed to 4.5 volts at wide-open throttle position. It may be necessary to also check the signal voltage at the PCM to determine if there is a voltage difference between the sensor and what the PCM reads. The throttle position sensor signal should also be checked with a graphing multimeter or oscilloscope to determine if the voltage increases smoothly without interruption throughout the entire range of motion. If an advanced scan tool is available, the position sensor is usually displayed as a percentage of the actual throttle position; ensure the desired position value is similar to the commanded position value.
Throttle Actuator Motor – The PCM/ECM will send a signal to the throttle actuator motor based on the pedal position input and a predetermined output value depending on operating conditions. The pedal position is known as the desired input because the PCM/ECM controls the throttle blade position and may limit its performance under certain conditions. Most actuator motors are controlled by duty cycle. Check the correct resistance of the throttle actuator motor by disconnecting the harness connector using the DVOM set to the ohms scale with the positive and negative leads at each end of the motor terminals. The resistance should conform to factory specifications; if it is too high or too low, the motor may not move to the commanded position.
Check the wiring by testing the power supply using a factory wiring diagram to locate the appropriate wires. The power wire can be checked using the DVOM set to the volts scale with the positive lead on the power wire and the negative lead on a known good ground. The voltage should be close to battery voltage with the key in the on or run position; if there is a significant power loss, the wiring may be suspect and should be traced to determine where the voltage drop occurs. The signal wire is grounded through the PCM and turned on and off by a transistor. The duty cycle can be checked with a graphing multimeter or oscilloscope set to the duty cycle function with the positive lead spliced into the signal wire and the negative lead on a known good ground – a standard voltmeter will only display an average voltage, which may make it more difficult to determine if there are voltage interruptions over time. The duty cycle should match the percentage commanded by the PCM/ECM. It may be necessary to check the commanded duty cycle from the PCM/ECM using an advanced scan tool.
Throttle Body – Remove the throttle body and check for any obstructions or buildup of dirt or grease around the throttle blade that would prevent normal movement. A dirty throttle blade can prevent the throttle blade from responding properly when commanded to a certain position by the PCM/ECM.
PCM/ECM – After checking all other functions of the sensors and motor, the PCM/ECM can be checked for desired input, actual throttle position, and commanded motor position using an advanced scan tool that will display the input and output as percentages. If the values do not match the actual numbers obtained from the sensors and motor, there may be excessive resistance in the wiring. The wiring can be checked by disconnecting the sensor harness and the PCM/ECM harness using the DVOM set to the ohms scale with the positive and negative leads at each end of the harness.
It will be necessary to use a factory wiring diagram to locate the appropriate wires for each of the components. If the wiring has excessive resistance, the numbers displayed by the PCM/ECM may not match the desired input, target output, and actual output, and the trouble code will be set.