B1845 code engine won't start every time, esp after sitting for 1 hr on 2000 Ford Expedition

2000 Ford Expedition won't start all the time, esp after I
've been out of truck for 45 min or so. lights ok, battery ok, just doesn't start, diagnostic code B1845. I tried entering my key manually lock and unlock. no solution here.
Help me, please, Momma needs to get kids to school, dance, and grocery shop.