I haven't seen the answer to this anywhere so far, so I'm asking it here - and thanks in advance for any and all answers. How sensitive is the TPMS to drop in tire pressure - i.e. how many lbs./sq. inch does the pressure have to drop before a warning kicks in? I rechecked my tire pressures today, after a month, and they were all down 4 lbs. - but no warning or signal.