A P0171 (System Too Lean - Bank 1) code is set on an OBD-II vehicle. What should be checked FIRST?

When a P0171 code is triggered, it indicates that the engine's air-fuel mixture is too lean on Bank 1, meaning there is not enough fuel being supplied for the amount of air entering the engine. The first step in diagnosing this issue should be to check the fuel pressure.

Fuel pressure is crucial for the correct operation of the fuel injection system. If the fuel pressure is too low, it can lead to an insufficient amount of fuel being injected into the engine, which would cause the lean condition that the P0171 code signifies. By ensuring that the fuel pressure falls within the manufacturer's specified range, you can eliminate one of the primary causes of a lean condition right away.

If the fuel pressure is found to be adequate, then subsequent checks can be performed on components like the mass airflow sensor, oxygen sensors, or the intake air temperature sensor. Each of these parts can affect the air-fuel mix, but verifying fuel pressure first helps pinpoint whether the problem is indeed fuel-related. This methodical approach is essential for effective troubleshooting and repair.