Webasto heat sensor settings, it it faulty ?

[The big slipper]

Hi Guys,
does anyone know how accurate the +/- is on a Webasto diesel heater sensor is, when it's set at 20 degC in the van it's heating up to 25 degC, then stopping, but cools down to around 15 degC before starting up again, could do with it controlling a bit better.
Thanks

 

[Soycd]

Our heat sensor had a small plastic cover/protector which I’ve removed. This seems to help the air get to the sensor.

 

[The big slipper]

Our heat sensor had a small plastic cover/protector which I’ve removed. This seems to help the air get to the sensor.

thanks, I'll give it a try.