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OBD II fault code P0171 is a generic code that is defined as P0171 – “System Too Lean Bank 1”, and is set when the PCM (Powertrain Control Module) receives input data from the #1 oxygen sensor or air/fuel ratio sensor on bank 1 indicating that the air/fuel mixture has been too lean for an extended period of time- usually about 20 consecutive seconds.
Note that “Bank 1” refers to the bank of cylinders that contains cylinder #1, and that “#1 oxygen or air/fuel ratio sensor” refers to the sensor that is located upstream of the catalytic converter on Bank 1. In a fully functional system, the upstream oxygen sensor or air/fuel sensor directly measures the amount of oxygen in the exhaust stream as it leaves the engine as a means for the PCM to control the air/fuel mixture.
On gasoline engines, this mixture should ideally consist of 14.7 parts of atmospheric air to one part of fuel, which ratio is known as the “ stoichiometric ratio”, expressed as 14.7 : 1.
On engines that use oxygen sensors, this ratio will be expressed as a variation on most scanners; for instance, 15.5 : 1 when the mixture is too lean, while on engines that use air/fuel ratio sensors, the ideal stoichiometric ratio will be expressed as “Lambda 1” with deviations from the ideal ratio being expressed as a fraction of “1” on most scanners if the mixture is too lean.
In practice, while oxygen sensors and air/fuel ratio sensors work on vastly different principles, both types of sensors perform the same function. When the exhaust stream passes over the sensing element, the sensor generates a signal voltage that is directly proportional to the amount of oxygen in the exhaust gas.
Thus, when the sensor detects a lean-running condition for whatever reason, it signals this fact to the PCM, which then takes corrective action by making suitable adjustments to the fuel delivery strategy, ignition timing, and sometimes to the configuration of the variable valve/cam timing system.
In a fully functional system, these adjustments are performed several times per second, and all faults that prevent the PCM from controlling the air/fuel mixture effectively will result in dedicated codes being set.
However, in some cases, such as where the air/fuel mixture is being influenced for an extended period of time by for example, unmetered air entering the engine through vacuum leaks, the PCM’s ability to correct the mixture is limited, and it will set code P0171 and illuminate a warning light as a result.