I was told by a mechanic that the sensor light on my dash indicating low coolant might be a defective sensor that is part of the resevoir. I have on several occasions checked the coolant level and he did as well and it was never really lower than a cup or two yet the sensor light has never gone off. now the when the mechanic checked it he also told me that in order to change it the entire resevoir has to be changed because the sensor is part of the unit. Is this true? and how do I go about changing this out?