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 and 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 and its limits and what happens when something goes wrong.
- Sometimes this means checking the system against one that is known to be working properly.

Know the History of the System
- How old or new is the system?
- What kind of treatment has it received?
- Has it been serviced in the past in such a manner that might relate to the present condition?
- What is the service history?

NOTE: A clue in any one of these areas might save time.

Know the History of the Condition
- Did it start suddenly or did it appear gradually?
- Was it related to some other occurrence like a collision 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
- Look for the simple rather than the complex.
- For example:
- Electrical conditions usually occur at connections rather than in components.
- An engine no-start is more likely to be caused by a loose wire or some component out of adjustment than a sheared-off camshaft.
- Know the difference between impossible and improbable. Certain failures in a system can be improbable and still happen.
- New parts are just that -new. It does not mean they are always well-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 does not correct the original condition.

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