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The Code Reader


Diagnostic Trouble Code: P0171 System Too Lean Bank 1, P0174 System Too Lean Bank 2

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DTCs P0171 and/or P0174 are stored when the control unit recognizes that there is too much oxygen in the exhaust based upon readings from the oxygen sensor(s).


In order for complete combustion to take place, the air/fuel ratio must be at the stoichiometric ratio of 14.7:1. The control unit monitors the reading of the oxygen sensor(s) or the air/fuel ratio sensor(s) to determine if the engine is running lean (too much air) or rich (too much fuel.) The control unit will constantly adjust fuel delivery to keep the mixture at the stoichiometric ratio. This constant, rapid adjustment can be seen on a scan tool as short term and long term fuel trim.


More to know:

DTCs P0171 and P0174 are the same code except for the fact that they represent a different bank of cylinders. A vehicle with an inline engine will only indicate P0171, and if the engine is a V- or opposed configuration, it can indicate either code, or both at the same time, depending on the cause of the problem.


In many cases, your customer will not notice any driveability problems and their only concern will be an illuminated MIL, however it is possible that they may indicate rough running, poor performance, low power or other similar concerns.


A vacuum leak is a common cause of this problem, but don’t approach your diagnosis with this as your prevailing thought. Be sure to think of a lean condition as two-sided, being caused by either too much air or not enough fuel.

Check all possible sources for a vacuum leak or unmetered air including the intake system, vacuum hoses, PCV system and exhaust system. Don’t forget that a smoke machine may be the only way to locate some leaks.

Check fuel pressure and volume; low fuel pressure or a plugged filter can cause this problem. Check fuel injector operation; a dirty fuel injector can also cause this problem.


Monitor long- and short-term fuel trim using a scan tool and check MAF sensor operation as well as oxygen sensor operation.

Before you begin:

Always look for manufacturer information and TSBs. As with all computer-related diagnostics, the possibility of reprogramming or an electrical problem exists. Manufacturer information can save you a lot of time and trouble.

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