Warranty for service includes 90 days parts and labor. Spring warranties cover parts and labor for two years. Warranty coverage is based on normal use and does not cover abuse or acts of God. We normally recommend replacing both because they normally wear at the same rate, and they usually break within six months of each other. They wear out like car tires, only you can't tell from looking at them when they are ready to break.
Springs break because every time the door opens and closes, the spring wire bends a little.
You may have broken a paper clip or a piece of wire before by bending it back and forth. This is what happens to springs. If the spring that is not broken has been replaced in the previous year or two, we do not recommend replacing it. If the other spring looks like it has been replaced, but no one knows, I usually ask the custome to decide. Newer springs usually look black.