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