When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The level sensor is in the top of the radiator, just below the filler neck.
If it's not registering I suggest you replace it, though you might check it first. I believe it should show very low resistance when level is up and very high resistance when level drops below the sensor.