Multi-code analyzer
Scan tools hand you a list. A list is not a diagnosis. Codes that appear together often describe one fault from several angles — and which one you chase first can be the difference between a vacuum-leak repair and an unnecessary catalytic converter.
P0171 alone says bank 1 is running lean. Suspicion falls on that bank — its injectors, its oxygen sensor, a leak on that side.
P0171 with P0174 says both banks are lean at the same time. Two independent faults producing the identical symptom on both sides simultaneously is far less likely than one shared cause, so the priority shifts to what the whole engine shares: the mass airflow sensor, fuel delivery, a large vacuum leak, the PCV system.
P0420 with P0171 changes the order of work entirely. The catalyst monitor compares two oxygen sensors, and a lean mixture makes that comparison unreliable. Replacing the converter first can mean spending a four-figure sum on a healthy part.
That reordering is what this tool does. Paste your codes above.