Trouble with Flash and possibly metering in A+mode

[JuliaS]

I bought my Rebel t5i in Nov 2014, It has been working great.  Then about a month ago it started having trouble with the flash. It also doesnt seem to be reading the light correctly.  Sometimes the flash will go off and other times no and the pictures are coming out dark.  It tends to do a bit better on the P setting.  It will try to get the reading and then say busy.  At times the flash will pop up.  Any help?

[TCampbell]

It will say "busy" if it's working on focus.  If the camera is in "One Shot" focus mode then it willl not take a photo until it is able to lock focus on something.  If it's too dark, it may give up without being able to lock focus.

If you are referring to the builtin pop-up flash... the range is rather limited (it's not a very large flash -- which is why there are external flashes that can provide significantly more power.)  In A+ mode the flash is automatic in that if the camera thinks it needs it, it will pop it up.  In Program mode, it _will_ fire the flash if you pop it up and it will not use the flash if you don't pop it up (assuming we are talking about the builtin flash).

In a very dark environment or for subjects which are farther away than roughly 10', you probably want to use an external flash such as the 430EX II, the 430EX III-RT (the updated replacement of the 430EX II although the 430EX II is still available) and the flashship flash is the 600EX-RT.  All of these speedlite flashes can emit a "focus assist" beam.  It's a red pattern that the flash projects and the camera can use that pattern to lock focus (even in complete darkness.)  (BTW, the focus-assist beam is _not_ emitted if the camera is in AI-Servo mode -- so this isn't appropriate for action photography.)