"What is a Diagnostic?

A diagnostic is a series of tests by which you eliminate as good or bad the component you think is causing the problem. So, if the component (being tested) tests good, you then go on to the next component in the system and test that one. This process continues until the BAD part is found and then replaced. To put it simply: A diagnostic is just a process of elimination... and knowing this will give you a big testing advantage.
Now, many folks don’t have the time and/or inclination to learn how to test the problem on their vehicle and just want someone to give them a ‘guess’ (that’s right on the money) as to what the solution is to their car’s NO START. With a modern computerized fuel injected car or truck, this just isn’t possible anymore. So, I’ll add the following to dispel some common myths about what an Automotive Diagnostic is not:

• Connecting a Scan Tool and reading the Codes is not a Diagnostic.
• Telling a Mechanic or Technician the symptoms and receiving some suggestions as to the cause in not a Diagnostic.
• The surest way to throw money down the drain is to replace something based on the two above lines.

Ok, keeping the above in mind, the following testing tips and guidelines (in this article) can be modified to fit your particular need. Each No-Start Condition that you'll deal with will have a different thing causing it. And you're not going to find all of the tips and tricks here. The good news is that the testing tips in this article will give you a great head start.
Remember what I stated at the beginning of the article about Fuel and Spark? Well, to start this process of elimination as it relates to a NO-START CONDITION, we need to first find out if it's caused by a lack of FUEL or SPARK. How do we do this? With some very specific tools."