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Parts for Jenn-Air JCB2058GEW / PJCB2058GW0 Refrigerator - AppliancePartsPros.com

See the attachment for the tech sheet

There seems to be some confusion on this unit so first you have to determine if the unit uses a defrost timer as shown in the parts and the tech sheet Or have they updated the unit to use a defrost board.
Note that this would not be the first time a unit has been changed and they did not document it anywhere.

The timer is Item 12 in section 03.
There is usually a hole in the cover so that you can rotate the timer cam so take a look to see if you have one.

Your tech is correct that if the heater and defrost thermostat are OK then odds are that the defrost control device be it a board or a timer is the likely culprit.

If you have a timer you can manually rotate the timer cam until the unit enters a defrost cycle ( fans and compressor will shut off) if the defrost heater then gets hot the timer is bad.

One thing that does seem strange is that if the unit has been updated to a defrost board it is usually in the same location as the defrost timer was.

I am assuming that it is a defrost problem re: the evaporator coils are heavily iced/frosted over, the evaporator/freezer fan is running and the compressor runs all the time except when the unit goes thru a defrost cycle.