I need help from you refrigerator/freezer experts!?

I recently had the timer replaced on my very old GE refrigerator/freezer. (The freezer was not defrosting & was frozen up; and not cooling.) The heater is working fine. Everything else checked out. After 3 weeks, the unit is doing the same thing. Is it possible that the coils are dirty? It is a major undertaking to pull it out & clean it, but I will if you knowledgable folks think that is the problem. Thanks!