Samsung RS21DCNS freezer evap not defrosting

Hi, have problem of freezer evap icing up every few weeks, and will not do defrost cycle. Diagnostic panel shows defrost fault (middle bar, left digit).
Checked resistances in evap and drain defrost heaters (both ok), at 245 Ohms and 1250 Ohms.
Have continuity at thermal fuse, so that’s ok.
Have variable resistance at defrost sensor (it’s a white one – should it be black?), depending on temperature, so that seems ok (6 or 7 kOhms at lowish temp).
Fan is ok.
Would like to know what I can do next to find this fault ?????

Is there a way to force a defrost cycle via the control panel on front?

Thanks for your time, hope you can help!