Maytag Mzd2766gew. Fridge side not cooling. I have replaced the adaptive defrost board but still not defrosting - ice builds up on the coils in a few days. thermostat assembly or capacitor?

Sounds like a bad defrost terminator or bad heating element?
Fridge/freezer freezing up- and no fan or air circulation:

first check the defrost terminator thermostat it may have gone out. Thus not allowing the heating element to melt the ice.

AT TIMES The fan usually stops working AND CAN MAKE FUNNY NOISES AS WELL due to ice obstruction when the defrost terminator has gone out and u get ice build up? u can check the terminator with a multi meter for continuity but do not check at room temperature. stick the terminator in cup of ice water for 20 minutes then check it for continuity, should be closed when cold and open at room temperature.

May also need to check the defrost timer, The defrost timer is usually found behind the front grill "toe kick" of the refrigerator. Or in the fridge compartment near middle top section.

u can get by and test manually by placing a small screw driver into the tiny slot or hole underneath the timer and turn slowly clockwise till u hear 1 click. this will engage ur defrost cycle in about 15 minutes u should be able to come back and feel the heat in freezer as it melts the ice providing ur terminator is not bad?

Another way to test it is with a multi meter. Remove timer it from fridge and you will see 4 prongs numbered 2,1,4,3 in that order. Place ur meter prong on prong 1 & 3 (if your fridge has a capacitor wired in series with the windings, then u check for micro fares rather than OHMS) If u cannot confirm this test, still try the next.

Take ur meter prong and connect to prong 1 & 4, it should read resistance or ohms ( closed) now remove prongs and place on prong 1 & 2, this should now read infinity or open (no ohms) now take a small screwdriver place it in the tiny hole or slot under the timer. Turn it clockwise slowly till u hear 1 click. Now ur 1& 2 should read closed ( showing ohms or resistance) now place meter prongs on 1 & 4 and you it should now show open ( or no resistance - no ohms) this is how u know ur timer is good, Remember when testing for ohms ADJUST meter to use a high ohms rating in the thousands as the resistance is very high in OHMS. And adjusting to low ohms rating may give a false reading or not show the proper resistance..

Another item to check with a meter is the heating element itself. Located under the evaporator coils. It should show continuity or resistance OHMS WHEN TESTED AT BOTH ENDS OF THE PRONGS .

Lastly check the cold control thermostat with a meter for both OHMS and volts ( VOLTS is a live test with fridge plugged in so be careful) It must show 110 volts. An open or thermostat that reads infinity will not allow the proper cooling and defrosting cycles to operate and may in fact stop anything from happening as if there is no power to the unit?

ALSO CHECK THE AIR FLOW VENT SYSTEM and THERMISTORS At times it can get stuck to far closed or open. Some units feature a digital electric adjusting vent system while others are manually set. Check for any ice build up or other obstruction in the vent damper, see if u can manually open or close it. Some times when u have the settings set to the highest it closes off one area while allowing to much cold air flow to the other area!

THERMISTORS in the freezer and fridge section for continuity OHMS and for amount of OHMS current being put out. In most models around 1300 ( give or take 150 ohms) ohms is required

https://www.youtube.com/watch?v=zmdsBkO46dk defrost therm.

https://www.youtube.com/watch?v=6SJN8oaA2XE

https://www.youtube.com/watch?v=buX9KcZo2c0

https://www.youtube.com/watch?v=gVXT7pZyk_o

https://www.youtube.com/watch?v=53ZFeiWgtuk

God is so good: so this is why I give free advice so please thank him not me.