* Vehicle Speed Sensor (VSS): This sensor tells the transmission's computer (PCM) how fast the vehicle is moving. If it fails, the PCM won't know the appropriate gear to select, resulting in no shifting or erratic shifting. This is probably the single most common cause of sudden shifting problems.
* Transmission Position Sensor (TPS) / Output Speed Sensor (OSS): These sensors monitor the transmission's internal workings. A faulty TPS/OSS can prevent the PCM from receiving accurate information about the transmission's position and speed, leading to shifting issues or a complete lack thereof. Sometimes these are combined into a single unit.
* Neutral Safety Switch: This switch prevents the engine from starting unless the transmission is in park or neutral. A malfunctioning neutral safety switch *could* indirectly cause a sudden inability to shift if it's interfering with the PCM's signal processing. This is less likely to cause a complete cessation of shifting than the VSS or TPS/OSS.
* Other less likely, but possible culprits: Problems with the wiring harness connecting these sensors to the PCM, low transmission fluid, or internal transmission problems (like a broken band or clutch) could also manifest as a sudden inability to shift. However, these are generally accompanied by other symptoms like slipping, noises, or fluid leaks.
Important Note: Diagnosing the exact cause requires more information and potentially a diagnostic scan tool. Simply replacing sensors without proper testing could be costly and ineffective. A professional mechanic with access to diagnostic equipment should be consulted to pinpoint the specific problem.
