A yellow flashing light indicates that a safety pre-check must be completed in order to properly arm and use the copter (note that this procedure is for your own safety).
To see what has caused Pre-Arm safety check failures.
To fix specific Pre-Arm check failures.
There is actually a way to deactivate these pre-arm safety checks (not recommended).
If you are confident that the pre-arm check failure is not a real problem you can disable the checks by:
Ideally however you should determine the cause of the pre-arm failure and if it can be resolved, return the Arming Check parameter back to “Enabled”
For more information about this please click here.