DTC is a clue, not a diagnosis

A diagnostic trouble code points you toward a system that is unhappy. It does not tell you which part is guilty, and it definitely does not tell you to throw a part at the car.

The useful sequence is simple: read freeze-frame data, confirm the customer's symptom, look at related codes and repair history, and do the fastest disproof test before you authorize anything expensive.

• Freeze frame tells you the operating conditions when the fault set.
• The symptom tells you whether the customer complaint actually matches the code.
• Related codes show whether the problem is the cause or just collateral damage.
• The disproof test is the fastest way to eliminate the wrong fix.