Coolant temperature sensor o-ring (8 X 14 mm), seals the fan switch to the lower radiator hose

Generally speaking this o-ring is one time use only and comes with a new coolant temperature sensor. If you remove the sensor from the lower radiator hose you are forced to replace the entire sensor since BMW does not sell this o-ring separately. FCP on the other hand is able to sell this o-ring as an alternative to replacing an entire sensor. This is the original style o-ring made from the OE green silicone material.