The foam filter in the probe tip should be replaced if it becomes plugged with water or oil or appears dirty. To replace the filter, twist off the tip protector (counter clockwise) and simply push out (with a paper clip or similar object) the filter from the sensor side (larger end) of the tip protector. Slide the new filter into the small hole in the tip protector, then replace the tip protector. If the sensor becomes unseated when you remove the tip protector, take it out and plug it back in.