If a fault (open, short, over-current..etc) is detected on any of the driven lamp outputs required for signaling (which will be different for each vehicle region), then the frequency of the flashing signal sent by the BCM to the turn signals is intentionally doubled in speed to indicate an issue.

The vehicle can only determine if the issue is for the left or right indicator. It cannot determine if the issue is in the front or rear indicator, so they both must be checked.