Hi,
I don't think there is anything "optical" in the coolant sensor. It is simply a magnetic reed switch that is activated by a magnet embedded in a float. Possibly the post that the float rides up and down on is gunked up. If that is the case, you'll have to remove the tank and fill it up with water and shake it VIOLENTLY to remove the crap. It will be difficult because there isn't a direct shot to that area of the tank...too many walls/chambers.
...Ed
