Code P0720 is one of the OBD codes that is activated when the Powertrain Control Module (PCM) detects erratic or insufficient input from the output shaft sensor (OSS). Such a code generally indicates ...

If your GM car/truck is a '96-and-later, it incorporates an OBD-II-based PCM, which controls the transmission as well. For engine control, the PCM takes many sensor inputs (O2, TPS, RPM, MAF, MAP ...