Thanks for the link. @casinada
seems like A,B,C,D registered were already containing stored data's. So when i tried to do a factory power on settings i was getting "cannot overwite saved display" message.
Solution: press save in display storage and then press each registers until all red and green lights across each registers are turned off.
then again try to do the factory default power up settings(utility menu - stored settings/displays - factory default power-up). This
this time i dint get the error message.