usually the evaporator doesnt just go out.
BUT, if there are issues with it, the problems could be a dirt or debris plugged core, cracked or broken case, or a leaking seal or o-ring. also check to make sure the fins in the evaporator arent clogged with lint and hair, etc that would restrict air flow through it, decreasing its heat absorbing capabilities.
if its the expansion valve, there will be high pressures on the low side. usually caused because the remote bulb isnt making contact with the outlet of the evaporator tube. make sure there is a good mechanical connection between the two and is secured properly. if thats correct, then the expansion valve is defective.