How do we diagnose complex "issues" when they go wrong?
Often, when an appliance stops working, we don't even think about repair; we simply purchase a new one -- there is limited or no diagnosis involved. We may check to make sure everything is plugged in, that a circuit breaker hasn't tripped, or even check the internet to see what we find, but that's typically the extent of it. Think about it. For example, many microwave ovens can be replaced for less than $100, while the average cost of a repair (according to Angie's List) is about $133. That's because repairing one typically takes a technician up to an hour to diagnose and repair the problem. Plus on top of the cost for the technician's time, there is the cost of the replacement parts. Combine this with the sheer inconvenience of the repair process like schedule coordination or shipping, and it becomes clear why this type of repair process only works for more expensive items.
When our gas fireplace stopped working, we called a company that specializes in fireplace service and repair. The process was interesting because it brought to light an interesting concept of what they consider "diagnosis." For this particular company, "diagnosis" was simply "calling the manufacturer hotline and having them identify what should be replaced."