While performing an ILS approach with autopilot engaged in approach mode, if I try selecting the heading button it doesnt work. When I get down to a certain altitude and I want to circle to land on differnet runway and I want autopilot to turn in heading mode it wont select. I have to disengage autopilot and then it wont even re-engage. Whats up?

You can't engage heading select while in approach mode !!( Maybe worth noting but the APP mode is the only AFDS mode that can handle Thrust, Pitch and Roll at the same time. )Just a bit of Boeing trivia for you. Fred.

Once APP is engaged the only means to disengage it is by turning off both FD's or disengage the AP.FinnJ