What does it mean?
OBD II fault code P0723 is a generic code that is defined as “Output Speed Sensor Circuit Intermittent”, and is set when the PCM (Powertrain Control Module) detects an intermittent signal from the sensor that monitors the rotational speed of the transmission’s output shaft. In many automatic transmission designs, the speed at which the output shaft rotates is the primary source of the input data the PCM or TCM (Transmission Control Module) uses to determine appropriate shift points.
However, this information is not used in isolation, or on its own. For instance, the PCM/TCM compares this data with input data from a sensor that measures the rotational speed of the input shaft as a means to detect transmission slippage or slippage of the torque converter lock-up clutch.
Thus, if everything in the transmission works as expected and there is no slippage present, the PCM/TCM can calculate shift points based on the rotational speed of the output shaft, as well as which gear the transmission is in without reference to the position of the gear selector.
Nonetheless, although the PCM/TCM uses input data from the output shaft speed sensor as a primary source of input data to determine appropriate shift points, the output shaft speed sensor on most, if not all vehicles and transmission designs also does additional duty both as a vehicle speed sensor and a transmission fluid temperature sensor.
In these cases, the output shaft speed sensor also “drives” the speedometer in the instrument cluster, which represents another source of input data to the PCM/TCM with which to calculate shift points. In terms of operating principles, most (if not all) transmission output shaft speed sensors are simple Hall-effect sensors that generate an electrical signal each time a raised point on the rotating output shaft passes in front of the sensing element.
In fact, this is similar to how wheel speed sensors, crankshaft position sensors, or camshaft position sensors generate signals in conjunction with reluctor wheels. Depending on the transmission design, the PCM/TCM “counts” the number of signals that represent one complete revolution of the output shaft.
Thus, since the control module “knows” how many signals should occur within a set period, based on input data from other sensors that include the input shaft sensor, the turbine speed sensor, and the engine speed sensor, it can verify that the output shaft is rotating at the correct speed based on current operating conditions. If this value agrees with what the PCM/TCM expects to see, it will continue to control and manage gearshifts normally.
If, however, the output shaft sensor relays input data to the PCM/TCM that is not consistent, in the sense that the signal is intermittent or fluctuates over a predefined period, the PCM/TCM will recognize that it cannot control the transmission effectively. When this happens, the PCM/TCM will set code P0723 and possibly illuminate a warning light.
Note that in some cases, the PCM/TCM may also initiate a fail-safe or limp mode both as a safety precaution, and to protect the transmission against mechanical damage.