What does P0171/P0174 indicate in OBD-II codes?

• A. System too lean in banks 1 and 2, respectively.
• B. System too rich in banks 1 and 2, respectively.
• C. Cylinder misfire recorded in banks 1 and 2.
• D. Oxygen sensor failure in banks 1 and 2.

Answer: (A)

P0171 and P0174 mean the engine is running too lean on both sides of the engine, with Bank 1 and Bank 2 showing higher air-fuel ratios than normal. A lean condition means there’s more air than fuel in the mixture, which the PCM detects through the oxygen sensors and then tries to compensate by increasing fuel via fuel trims. When both banks show lean readings, it points to a common, system-wide cause rather than a single-cylinder issue—things like a vacuum leak, a faulty mass air flow sensor, or inadequate fuel delivery.

Think of it this way: the oxygen sensors see more oxygen in the exhaust than expected, so the computer adds fuel to try to correct it. If the trims max out or can’t bring the mixture back to normal, the codes set. To diagnose, check for vacuum leaks in the intake and hoses, inspect the mass air flow sensor for dirt or failure, verify fuel pressure and delivery (pump, filter, regulator), and look for exhaust leaks before the downstream sensors. Also ensure there aren’t clogged air filters or faulty PCV valves contributing to too much air being drawn in. After fixes, clearing the codes and test-driving helps confirm the lean condition is resolved.