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P0172 System too rich code caused by K&N or other oiled filters (contaminated MAF)

October 14, 2019

The P0172 check engine code is an interesting code because it identifies that the engine is running rich, but doesn’t otherwise specify the root cause.

First thing to check and clean when such a code occurs is the mass air flow sensor. Generally it is the first sensor to become fowled, and the contamination will cause the sensor to under estimate the amount of air that is entering the engine.

What the code in laymen terms is trying to tell us is that the amount of fuel the MAF thinks the engine needs, isn’t matching the actual needed amount as it is read on the backend by the O2 sensor.

The MAF sensor makes a prediction by the air volume entering the engine, to determine what volume of fuel is needed to achieve a clean efficient burn; the Oxygen Sensor (O2 Sensor), confirms the cleanliness of that burn.

The two sensors work together in a loop to adjust the air/fuel accordingly. However, when the two sensors fall out of sync, and can’t make sense of each others readings, it will default to displaying a check engine code pertaining to the condition the O2 Sensor is reading, either rich (not enough air) or lean (too much air). These two codes are the aforementioned P0172 for rich and PO171 for lean.

The reason the code doesn’t identify a root cause (eg. improbable MAF reading), is because the system doesn’t know why its not getting enough air (in the case of P0172), it could be a dirty MAF, but it could also be a clog in your intake, or a dirty filter, or a dirty O2 sensor.

One of the biggest overlooked causes for a dirty MAF and therefore the P0172 code are oiled lifetime filters like the K&N filters. Oiled filters are just that, oiled, factory filters are dry. What ends up happening is that the oil gets sucked into the intake and splashes onto the MAF wires that are used for telemetry.

These wires work under the same general principle, they heat up and calculate the amount of air flowing by the rate that it cools down.

As you can imagine, if you put oil or contaminants on this wire, the rate at which it will cool will be different and therefore the reading will be incorrect.

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