Air Conditioning - central a/c heating system Frozen coils

I have frididiare central a/c heating system ( air handler and outside fan unit).
I noticed the outside unit staying on continiously while the thermostad is shut of. I've had to disingage the fuses on the outside unit to turn it off, this is when i noticed the coil frozen that is conected to the air handle in my basement.
I traced the pipe all the way to the air handler opened the unit and realized the coils were completly frozen.
It seems that even after the coils are thawed and the unit turns on by the thermostad, it freezes up again.

I have clened the air filter checked the coils for any abstructions dirt they all look clean; this is a fairly newly installed unit 3 years old...

any one know what the problem is? How do i fix this with out having to call in a service rep?

help...

Is this a heat pump ???or just a AC unit. From what you say the contactor out side can be stuck. If it runs and the tstat dont call for it. Or do you have a short in the tstat wire.Info Info helps just what all do you have there for heat and cool.
Can be low on freon .Defrost board bad.Then you need a tech

Whats the temp where you are