Diagnostic Theory

Diagnosis is more than just following a series of interrelated steps in order to find the solution to the specific condition. It is away 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 these rules are followed, the cause of the condition is usually found the first time through the system.

Know the System
^ Know how the parts go together.
^ Know how the system operates as well as its limits and what happens when the system goes wrong.
^ Sometimes this means checking the system against one that is known to be working correctly.

Know the History of the System
A clue in any one of these areas may save time:
^ How old or new is the system?
^ What kind of treatment has it had?
^ Has it been repaired in the past in such a manner that might relate to the present condition?
^ What is the repair history?

Know the History of the Condition
^ Did it start suddenly or appear gradually?
^ Was it related to some other occurrence such as a collision or previous part renewal?
^ Know how the condition made itself known; it may be an important clue to the cause.

Know the Probability of Certain Conditions Developing
^ Look for the simple rather than the complex.
^ For example:
- Electrical conditions usually occur at connections rather than components.
- An engine no-start is more likely to be caused by a loose wire or small adjustment rather than a sheared-off camshaft.
^ Know the difference between impossible and improbable. Certain failures in a system can be improbable but still happen.
^ 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
Lowering the pressure in a front tire may correct the condition of a vehicle leaning to one side, but it does not correct the original condition.

Be Positive the Cause is Found
^ Double check the findings.
^ What caused a worn component?
^ A loose transmission or engine mount could indicate that other mounts are also loose.