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. If you check closely, you will find that all good technicians use the same basic procedure.

There are basic rules for diagnosis. If you follow these rules, you will usually find the cause of the condition on 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 you are working on.

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 all of a sudden, 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 eventually find out the failures were just "improbable" and actually had happened. Something to remember is that new parts are just that - new. It does not mean they are always good, functioning parts.

Do Not Cure the Symptom and Leave the Cause
For example, lowering the air pressure in a front tire may correct the condition of a vehicle leaning to one side, but it does not correct the original problem.

Be Positive the Cause Is Found
Do not be fooled into thinking you have found the cause when you have not. 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.

Diagnosis Charts
Charts are simply a way of expressing the relationship between basic logic and a physical system of components. They help corner the cause of a condition in the shortest possible amount of time. Diagnosis charts combine many areas of diagnosis into one visual display:
- Probability of certain things occurring in a system.
- Speed of checking certain components or functions before others.
- Simplicity of performing certain tests before others.
- Elimination of checking huge portions of a system by performing simple tests.
- Certainty of narrowing down the search to a small portion before performing in-depth testing.

The fastest way to find a condition is to work with the tools that are available, which means working with proven diagnosis charts and the proper special equipment for the system.