What does it mean?
SPECIAL NOTES: Take note that when diagnosing code P0721 – “Output Speed Sensor Circuit Range/Performance”, non-professional mechanics are generally limited to a few basic troubleshooting and repair steps. On almost all modern automatic transmissions, this particular code can be caused by failures/malfunctions in control modules and/or circuits and systems that may at first glance not be directly related to the transmission output shaft speed sensor or its control system.
Therefore, the information in this guide is intended for general informational purposes only, and should NOT be used in ANY diagnostic/repair procedure for this code on ANY application without making proper reference to the manual for the affected application. Thus, if the information provided here does not resolve the problem, the better option is to refer the vehicle to the dealer or a specialist transmission repair shop for professional diagnosis and repair. END OF SPECIAL NOTES.
OBD II fault code P0721 is a generic code that is defined as “Output Speed Sensor Circuit Range/Performance”, and is set when the PCM Powertrain Control Module) or TCM (Transmission Control Module) detects an abnormal or unexpected voltage or signal from the transmission output shaft speed sensor or its control circuit. Note that this code only applies to applications with automatic transmissions.
Also note that on some applications, the transmission output shaft speed sensor provides input data for the vehicle speedometer. The primary purpose of the transmission output shaft sped sensor is to provide the PCM/TCM with input data on the rotational speed of the output shaft. The PCM/TCM uses this data (which is compared with input data from other transmission speed sensors) to calculate shift strategies, as well as the desired hydraulic pressure required for each gear ratio.
Based on the total body of input data from all implicated speed sensors, but especially on input data from the output shaft speed sensor, the PCM/TCM controls the shift solenoids to accomplish appropriate gear shift points and patterns.
From the above it should be clear that if the PCM/TCM is deprived of accurate input data from the transmission output shaft speed sensor, the control module is unable to control gear shifts effectively since it is unable to direct hydraulic pressure to the correct shift solenoid at the right time. The image below shows a typical transmission output shaft speed sensor. Note that depending on the application, these sensors may have two, three, or four wires in their connectors.