If you are unfamiliar with the process of reading HVAC, or heating, ventilation, and air conditioning wiring diagrams, then fixing your own furnace may seem like trying to read Greek or Egyptian without some kind of translator. However, if you learn a few basic symbols and terms, you will be able to read HVAC wiring diagrams fairly easily. One important thing to understand is that the wiring diagrams have their own special symbolism that covers each specific part in the system, as well as a color coding that may or may not be the same depending on the year or model of the furnace that you have.
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Review the symbols used by the company whose furnace you are fixing. Each company may have their own specific symbols and terms for things. For example, some companies may call an electronic part a transformer, and others will call the same part a starter. Whatever company you are working on, they will have a key or other symbol diagram in place, usually printed on the wiring diagram itself. Check this for manufacturer specific codes and names for things.
Follow the color coding. Most manufacturers use color-coded wires to help narrow down problem spots. Along with the key that shows the manufacturer specific symbols and terminology, your HVAC wiring diagram should also have a color-coded key at the bottom that will tell you that the purple wire powers the inducer motor or that the red wire goes to the thermostat. While there are some standards, most companies have their own color coding. A shop manual may come in handy here.
Follow the flow of electricity. Electrical flow is shown in wiring diagrams by an arrow that is superimposed on top of the wire. If the arrow is pointing in one direction, electricity flows from a starting point on one side of the arrow to an ending point on the other side and does not reverse itself. This way you can track where the electricity stops flowing to determine the problem.
Use an electrical multi meter to verify diagram accuracy. If a resistor is shown in the wiring diagram and specified to have 1 to 3 ohms of resistance, you will want to check this with your multi tool to make sure that the part itself is in spec.