We have been installing aftermarket as well as OEM seat heaters for years, What happens is that once they reach an optimum heat range they remain there. The reason you feel they shut of is that your bum and the heaters are the same temp and they remain together. These heaters just have a wire element in them and they only acheive such and such a temp and do not continue to get warmer. Most have two settings high and low, obviously the high gets a little warmer a little faster. Tada.