2and70. I dont mean to sound rude but are you a 737-rated pilot? If you are its time you got back into the books and have a sim session mate! Your asking a lot of basic questions that you should already know, are very unsure of yourself and a few of your statements are pretty inaccurate.
As any properly endorsed 737-EFIS pilot will tell you, the only time both FD bars will disappear is when in a LVL CHG climb with a commanded speed equal to minimum speed and a minimum rate of climb cannot be maintained without decelerating. The AT will disengage itself too.
The other times all the FD bars will automaticaly disappear is when:
* ILS signals become invalid
* FLARE annunciates (green)