To provide an in-vehicle electronic control device that properly analyzes a cause of a predetermined vehicle behavior on the basis of current vehicle traveling information, and to provide a diagnostic tool and a diagnostic system.

On the occurrence of a predetermined vehicle behavior, where a throttle opening is large although an accelerator is not pressed (S400: Yes), the electronic control device stores current vehicle traveling information, time and storage execution information indicating that the vehicle traveling information has been stored (S402). When an engine is stopped (S404: Yes), the electronic control device executes failure determination on the operation of a CPU (S408) when determining that the vehicle traveling information has been stored on the basis of the storage execution information on behavior information (S406: Yes), stores a failure determination result, failure determination execution time and determination execution information indicating that the failure determination has been executed, as failure determination information (S410), and cuts off power to be supplied to the CPU (S412).