What does it mean?
OBD II fault code P0170 is defined as “Fuel Trim Malfunction (Bank 1)”, and is set either when the PCM (Powertrain Control Module) experiences difficulty in controlling/maintaining the ideal air/fuel mixture, or receives a fuel trim request from the post-catalytic oxygen sensor that exceeds the PCM’s ability to adjust the fuel trim to meet the requested level of adjustment over a specified period of time. Bank 1 refers to the bank of cylinders that contains cylinder #1.
Fuel trim is the action taken by the PCM to adjust, or maintain the correct air/fuel mixture during normal engine operation. As the engine speed/load changes, the post catalytic oxygen sensor (oxygen sensor #2) continuously requests that adjustments to the air/fuel mixture be made to maintain maximum engine efficiency.
Fuel trim requests can be either negative or positive; negative trim is an adjustment to make the air/fuel mixture leaner, while positive fuel trim is an adjustment to enrich the air/fuel mixture. Typically, code P0170 relates to a condition in which long term fuel trim requests cannot be met, meaning that when the requested trim value exceeds the PCM’s ability to adjust the fuel trim to the desired level by more than about 25% a code will be stored, and the CHECK ENGINE light will be triggered.
Differences smaller than about 25% between the requested trim value and what the PCM is able to achieve are known as short-term fuel trim requests, and are much less critical to efficient engine operation. As such, these requests are far less likely to cause a code to be stored. It is worth noting that code P0170 rarely occurs on American or Japanese-made vehicles.
Vehicles most plagued by this code are of European manufacture, and most notably BMW, Audi, Volkswagen, and Mercedes-Benz models that are notorious for their high oil consumption rates. On these models the root cause of code P0170 can almost always be traced to the Mass Airflow Sensor. The image below illustrates how the pulse width, which is the amount of time (in milliseconds) the injector stays open to inject fuel is adapted to compensate for lean -, or rich-running conditions.
Using the grey vertical line as a median point, the injector pulse width is increased to the right (red), to inject more fuel, and decreased to the left (blue) to inject less fuel. The information on which these adaptations are made is derived from the post catalytic (#2) oxygen sensor.