P1684 is a computer diagnostic code on a vehicle that signifies that a battery failed to connect within the past 50 starts. Car dealers and mechanics refer to a large listing of OBD or on-board diagnostic codes to troubleshoot mechanical issues that need to be addressed on motor vehicles.

According to OBD-Codes.com, on-board diagnostics permit the monitoring of almost all the controls for an engine and a car's body and chassis. OBD codes not only allow drivers to spot repair troubles, they are able to track car speed and RPMs too.

The OBD system is in working order if the "Check Engine" light flashes momentarily when the ignition is started. The light should not stay on while the car is driven.