Check coolant level

As the title suggests, I have a problem with my coolant level or, do I?. Every time I start my 2005 td6 I get a warning from the dash, CHECK COOLANT LEVEL. There is always plenty in the tank. I've just changed the coolant level sensor. Still I'm being told to CHECK COOLANT LEVEL. Any ideas?.

Not the same model I know but every RRC I've owned had the same problem that just got worse, eventually I isolated the sensor & continued manual checks on the coolant level - just like those of my age always did