You need to have the codes read.  Keep in mind that just
because the light went out does not mean the system healed
itself.  That is an indication of a non-critical failure.
That does not mean it is not important and has no effect on
the system or to drive as long as you like before getting it
checked.  The fault code exist whether or not the actual
failure that set the code is constant.  A constant light
means you have a failed non-functional fault somewhere that
need immediate attention and the system may be in "limp"
mode to protect the system.  Either way, it needs to be
corrected ASAP as it will usually only get worse costing you
more money and polluting the air if left unattended.  In
many states, auto parts stores can read the codes for free.
Make sure you get the exact code if you post back here.  I
understand some states do not allow this practice for some
reason.

Lugnut

I am having a shifting problem with my mustang. A while ago my engine
light went off and it disappeared in 2 days before i could get some one
to check it. My car started to shift into 2nd gear in less time than it
use to. And couple of days ago it started the opposite. It will take
some good time to shift from 1st to 2nd gear. But rest seems to be
fine. Also if i accelerate steadily, it works fine and shift normal in
2nd -5th but if i accelerate fast, it would just increase the RPM and
won't accelerate.

-When its shifting.. its smooth doesn't look like its forcing into
the next shift.
-The Engine light is gone now.
-I had an error in oxygen sensor a while ago which was something like
delay in communicating with port A or C or something like that when I
hooked up the code reader from my friend.

Any one knows what can be the problem. Is it a bad sensor or bad
transmission??