Start with whatever you know. A diagnostic code is useful, but it is only one piece of evidence. Warning lights, heat, noises, smells and the way the vehicle runs can override a harmless-looking code.
These are qualitative escalation paths, not mileage or time guarantees. Mechanical failures do not follow a stopwatch.
A diagnostic trouble code identifies a condition the vehicle detected. It does not, by itself, identify the failed part. Vehicle-specific service information and the owner's manual take priority. If the vehicle has a safety-critical warning, fluid loss, overheating, braking trouble, steering trouble, fire risk or severe mechanical noise, use the conservative recommendation.
The rules in this prototype are deliberately conservative. Standardized DTC structure is based on SAE J2012; VIN identification uses NHTSA vPIC; hard shutdown rules are based on conventional manufacturer safety guidance for conditions such as oil-pressure loss, overheating and severe braking faults.