C7 Coolant Sensor Issue

Hi

I have Audi A6 Avant 3.0 tdi Quattro C7

I am getting a message when driving “switch off engine and check coolant level”

The red temperature symbol lights up also, but the temperature indicator on the left of the Rev counter stays normal (4 white bars)

The coolant level is fine, in between the minimum and max levels

Can anyone tell me is this likely to be the coolant level sensor or the coolant temperature sensor?

If I wanted to check the coolant level sensor, any link to instructions/guide for that?

Many thanks