What should I check on my vehicle before using the Code Reader to diagnose problems?

Before testing, check all fluid levels (engine oil, coolant, power steering, transmission fluid) and top off as needed. Inspect the air filter, engine belts, spark plugs, spark plug wires, battery terminals, and all rubber and steel hoses for damage or improper connections. Mechanical problems such as damaged hoses or wiring can cause 'false' fault codes, so fix known mechanical issues before performing any diagnostic test. (Page 12)