I wondered if anyone else had problems with the rotary knob not working on the 15 sec mode! If I turn the knob to 30 secs or test there is no 15 sec in between. If you press the knob very slightly upwards if will show 15 secs for s split second then as you release the pressure on the knob it either goes back to 30 sec or test mode. Seems like the knob has a slight flaw. Even when turning the unit off I have to apply slight pressure upwards on the point of the knob for it to enter the off position.