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When You Get a Diagnostic Code P0171

Vacuum gauge

When the engine's computer throws a P0171 code, this OBD-II refers to the oxygen level in the exhaust. Specifically, it is the "System Too Lean" code, referring to Bank 1 (the side of the engine exhausting from cylinder 1 on a V). It's triggered by the first O2 sensor, which will read too much oxygen in the air:fuel leaving the cylinders into the exhaust. The code basically means there is too much air in the system.

Most of the time, this means there's a vacuum leak in the manifold, allowing more air into the combustion chambers than the computer and engine want. This air is coming in outside of the intake system's control. Vacuum leaks can happen anywhere in the intake manifold after the mass airflow or similar sensor.

In our forums, user barnies1 had this problem on a Buick LaSabre. The diagnostics should begin with the basics and the most basic assumption is that this error code means you've got a vacuum leak or a bad MAF or PCV.

Many make the wrong assumption that this code means they have a bad O2 sensor. That's rarely the case and is a relatively expensive replacement that may or may not fix your problem. Better to test the fundamentals first.

Begin by checking for vacuum leaks downstream of the MAF and/or PCV. Learn how to do that here. Ultimately, this was the problem for barnies1.

Next, try cleaning the MAF. Then the PCV.

At this point, if your problem isn't solved, you can begin checking sensors and their circuits and then the fuel injectors and fuel system.