The quick test just runs a bunch of test before the car even starts. It will give you the codes that just occurred during the quick test and will also give you codes that have been stored in keep-alive memory. If you have a defective or dead system, the quick test should tell you what it is.
It can be stone cold to do the test. Timing and cylinder balance tests are another deal.
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351W 89 Mustang GT Convertible