Diagnosis of a driveability problem requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to begin extensive testing before completing the preliminary diagnostic steps. The preliminary or visual inspection must be completed in detail before diagnosis begins. In many cases this will shorten diagnostic time and often cure the problem without electronic testing.
This is possibly the most critical step of the diagnosis. Many fault codes or apparent failures are caused by loose, damaged or corroded electrical connectors. A detailed examination of all connectors, wiring and vacuum hoses can often lead to a repair without diagnosis. Performance of this step relies on the the skill of the person performing it; a careful inspector will check the undersides of hoses as well as the integrity of the hard-to-reach hoses blocked by the air cleaner or other components.
Wiring should be checked carefully for any sign of strain, burning, crimping or terminal pull-out from a connector. Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit. Pay particular attention to ground circuits, making sure they are not loose or corroded. Remember to inspect connectors and hose fittings at components not mounted on the engine, such as the evaporative canister or relays mounted on the fender aprons. Any component or wiring in the vicinity of a fluid leak or spillage should be given extra attention during inspection.
Additionally, inspect maintenance items such as belt condition and tension, battery charge and condition and the radiator cap carefully. Any of these very simple items may effect the system enough to set a fault code. The self-diagnostic system will not operate properly if the battery is low on charge.