Diagnostic Theory

DIAGNOSTIC THEORY
Diagnosis is more than just following a series of interrelated steps in order to find the solution to a specific condition. It is a way of looking at systems that are not functioning the way they should and finding out why. Also, it is knowing how the system should work, and whether it is working correctly.

There are basic rules for diagnosis. If you follow these rules, you will usually find the cause of the condition the first time through the system.

KNOW THE SYSTEM
This means know how the parts go together, how the system operates and its limits, and what happens when something goes wrong. Sometimes this means getting your hands on a system that is working properly and checking it against the one on which you are working.

KNOW THE HISTORY OF THE SYSTEM
How old or new is the system? What kind of treatment has it had? Has it been serviced in the past in such a manner that might relate to the present condition? What is the service history? A clue in any one of these areas might save a lot of diagnosis time.

KNOW THE HISTORY OF THE CONDITION
Did it start suddenly, or did it appear gradually? Was it related to some other occurrence, like an accident or previous part replacement? Know how the condition made itself known. It may be an important clue to the cause.

KNOW THE PROBABILITY OF CERTAIN CONDITIONS DEVELOPING
It is true that most conditions are caused by simple things rather than by complex ones, and they occur in a fairly predictable pattern. Be sure you know the difference between impossible and improbable. Many good technicians have spent hours diagnosing a system because they thought certain failures were impossible only to find out eventually that the failures were just improbable and actually had happened. Remember that new parts are just that - new. Being new does not necessarily mean that they are good, functioning parts.

DO NOT CURE THE SYMPTOM AND LEAVE THE CAUSE
Lowering the air pressure in a front tire may correct the condition of a vehicle leaning to one side, but it may not correct the original problem.

BE POSITIVE THE CAUSE IS FOUND
Double-check your findings. If you find a worn component or something out of adjustment, ask yourself: "What else could go wrong?" If one of the engine mounts was loose, could the other one be loose, too? What about the transaxle mounts? Build a picture in your mind of the relationship of the affected parts.