With our sophisticated Computer Scanners, we can communicate with the various CAN Bus Systems, and thus obtain additional diagnostic information.
These “codes” are, however, at best an indication, and not conclusions of root causes. Vehicle computers are not (yet) smart enough to diagnose and self-troubleshoot problems, at best they observe performance and compare to pre-set thresholds - if those thresholds are surpassed, they register the fault, and - if severe enough - turn on a warning indicator (MIL Light, Warning Message).
P1720 = TCM LOSS OF OUTPUT SPEED SENSOR (on a 2000 Jaguar XJ-8 VandenPlas with a ZF 5HP24)
As an example, a “Speed Sensor” error code in the transmission does not mean the speed sensor recognizes itself as being defective, but it does mean the computer sees abnormal data between input- and output speed sensors, and in turn records a “speed sensor” error.
Input speed is monitored by the TCM (Transmission Control Module on the CAN Bus) with a rationality check being made against the output speed. A fault will be flagged if the indicated input speed exceeds 7,400 rpm. Additionally, a failure judgment will be made, if the indicated input speed is <160 rpm with engine speed >608 rpm and output speed >224. The procedure is similar for the output speed. Under normal circumstances, after the output speed diagnostic fault code has been set, the TCM uses rear wheel speed information to compute its calculations. Should a second fault occur (in the ABS system, thus making rear wheel speed information unavailable) an additional fault code will be logged.
It is up to the Specialist to interpret the data, along with Test Drives, and other diagnostic information.