Whirlpool 6th Sense American Fridge Freezer not defrosting

Model S20DRBB32-A/G
Any help here would be much appreciated, as I’m pulling my hair out! Symptoms:
The 6th Sense light is constantly on ‘–‘ flashing for freezer temp, and fridge not cooling correctly.
Evaporator fan almost constantly on (at least 90{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} of time)

Ice make no longer working – no optics LED
Have had no control over temps on the display in months – the locking ‘key’ symbol always there and can’t get rid of it.
Evaporator coils totally iced over.Once I manually defrost coils – all works fine for 2 weeks, then same again.

Checked so far:
Thermostat on evaporator coils – ohmed out fine high and low temps.
Heater ok – has closed cct.
Fan working (per above)
Did have broken slider in baffle between freezer and fridge – changed out few months ago, and checked again as fine.
Have done the basic diagnostics test, where times down from 07 to 00……02 and 01 flash. I have seen different lists for what these codes stand for??
Also – I changed the main control board in the rear 3 years ago for a chattering relay……still have the old one if I need to swap parts across – if that’s even possible.

Question I have is – how do I manually trigger the DEFROST function, as I feel this is not happening probably because the main board is gone faulty???? Any ideas?
Thanks!!