Explained DTC: P0171 (system too lean bank 1) and P0174 (system too lean bank 2)


An internal combustion engine can only function at the top of its game when the air to fuel ratio is just right. When air and fuel get mixed together, there must be roughly 15 parts of air for every 1 part of fuel. The engine control unit is responsible for making sure the engine is working properly as it runs. This unit has a computer which communicates with other sensors and systems of the vehicle. This is how the unit can measure the amount of air and fuel which enter the internal combustion chamber and regulate it accordingly.

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Every vehicle has oxygen sensors which calculate the level of oxygen content which exists in the exhaust system. Based on the data, the engine control unit decides whether to inject more fuel or less fuel into the chamber. However, if the exhaust doesn’t have a lot of oxygen, this means the system is “running lean” because more fuel is being added to the chamber than needed. This will cause trouble code P0171 to be generated. Alternatively, if there is very little fuel and an abundance of oxygen in the chamber, it will cause trouble code P0174 to generate instead.

Trouble Code P0171

When you experience trouble code P0171 for the first time, you will be instantly warned with the Check Engine warning light on your dashboard. This will be followed by engine performance issues, including weak acceleration and other power problems. You may even experience misfiring on multiple occasions. Also, when your vehicle is stopped, and your engine is idle, the RPM may be too high or too low during the idling. Some of the causes associated with trouble code P0171 include bad mass air flow sensor, out of date software in the engine control module, or leaky vacuum.

Read also: Explained DTC: P0507 Idle Control System RPM Higher than Expected

Trouble Code P0174

When you have excessive amounts of air and very little fuel, then you will see trouble code P0174 come up. Some engine control units will attempt to correct this issue on its own by trying to turn the “running lean” condition back to a standard condition. But this will only happen in little amounts. Meanwhile, you will experience symptoms like bad fuel economy, rough idle, Check Engine warning light, weak acceleration, and engine stalling. The common causes of trouble code P0174 include bad fuel pump, clogged or dirty fuel filter, bad mass air flow sensor, clogged fuel injectors, or a bad oxygen sensor.


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