[COLOR="Blue"]How do I know if it is defrost thermostat or defrost heater or defrost timer or really blockage in the circulation?[/COLOR]
Since the compressor and the fans are running the unit is in cool down mode and not a defrost cycle. So as long as the evaporator/freezer coils are not clogged solid with frost/ice the unit it is not a defrost problem.
It should cool down to around 5 degrees F.
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I was surprised to see ice on evaporator within 2 hours after restarting the fridge.[/COLOR]
It is the frost pattern that is important. The coils should have a fairly even coating of frost. If the frost is just in one area (usually where the freon enters the coils) then it is a sealed system problem. The most common cause is low freon. This will be an expensive repair as you need a pro and often it makes more sense to put these dollars towards a new unit.

Below is a link take a look at the three example pictures about half way down the page.
http://www.applianceaid.com/ref...ator-not-cold-enough.php