feeling really dumb here, i got my thermometer last month and didn't get the chance to get used to it with our bfp. .now that my cycle has returned i took out again but the screen kept saying lo. i spent 3 days looking for a battery which was $4 would you believe only to change the thing and still get it saying lo!!
eventually it dawned on me to check the instructions and yep, that is what it does when you are supposed to start taking temp. at least when the battery actually does run out i have a spare ;-)