What is a good step-by-step diagnostic process for a check engine light?
Short answer: Verify the complaint, scan all modules and save data, check bulletins, form a plan from the codes and data, test before replacing, then repair and verify under the original conditions.
A repeatable process prevents parts-swapping. First, verify the complaint and talk to the customer: when does it happen, hot or cold, since when, any recent work. Second, run a full-system scan and save codes and freeze frame before clearing anything.
Third, check technical service bulletins and known fixes for the code on that vehicle. Many common faults have documented causes or software updates. Fourth, study the data. Look at fuel trims, sensor readings and the conditions in freeze frame, and decide what system test will prove or disprove your theory.
Fifth, test. Use a meter, scope, smoke machine, fuel pressure gauge or compression tester to confirm the fault in the circuit or component. Sixth, repair the root cause, not just the failed part, and ask why it failed. Finally, clear codes and verify the fix by driving under the original conditions and checking for pending codes and completed monitors. Document what you found for the repair order.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.