Basic Concept

Most important point to perform diagnosis is to understand systems (control and mechanism) in vehicle thoroughly.
It is also important to clarify customer complaints before inspection.
First of all, reproduce symptom, and understand it fully.

Ask customer about his/her complaints carefully. In some cases, it will be necessary to check symptom by driving vehicle with customer.

NOTE: Customers are not professionals. Do not assume "maybe customer means..." or "maybe customer mentioned this symptom".
It is essential to check symptoms right from beginning in order to repair a malfunction completely.
For an intermittent malfunction, it is important to reproduce symptom based on interview with customer and past examples. Do not perform inspection on ad hoc basis. Most intermittent malfunctions are caused by poor contacts. In this case, it will be effective to shake suspected harness or connector by hand. When repairs are performed without any symptom check, no one can judge if malfunction has actually been eliminated.
After diagnosis, make sure to carry out "erase memory". Refer to SELF-DIAGNOSIS .
For an intermittent malfunction, move harness or harness connector by hand to check poor contact or false open circuit.