Process of Elimination — Diagnostic Method
Systematic Elimination
Every diagnostic concern on any vehicle resolves the same way. Identify what the system is supposed to do. Confirm what it is actually doing. Find the gap between those two things. Work systematically from one end of the circuit to the other, confirming correct operation at each point until you find where correct stops and incorrect begins. The fault is at that exact point.
Never assume — always confirm
The most expensive diagnostic mistake is assuming. Assuming the fuel pump is good because it was just replaced. Assuming the ground is good because it looks clean. Assuming the code directly identifies the failed component. Codes identify the circuit or condition with the fault — not always the specific component. A P0340 camshaft position sensor circuit code can be a failed sensor, a wiring fault, a reluctor ring problem, or a timing issue. The code tells you where to look, not what to replace.
One change at a time
When testing, change one thing at a time. If you change multiple things simultaneously and the concern resolves, you do not know which change fixed it. You will potentially replace parts that were not the problem. Change one thing. Retest. Confirm whether it changed the symptom. Then decide what to do next.