P0171 – Fuel trim (FT) system too lean, bank 1

Trouble CodeFault LocationProbable Cause
P0171 System too lean, bank 1 Intake/exhaust leak, AIR system, MAFNAF sensor, fuel pressure/pump, injector(s), HO2S

What Does P0171 Mean?

The long term fuel trim and the short term fuel trim provide essential diagnostic data for diagnosing engine problems. In fact, a scan tool reading of the long term fuel trim can lead to a quick fix of a P0171 or P0174 code.

Fuel trims are settings which control the amount of fuel entering the engine to achieve a stoichiometric ratio of 14.7:1. The stoichiometric ratio is the volume mixture of air and fuel that will result in complete combustion of the fuel and oxygen. The car’s computer control will always seek 14.7 liters of air for each liter of gasoline. It is important to remember this fundamental law for your car.

Since the stoichiometric ratio is very important to engine performance, there are multiple air sensors and fuel controls that measure the air and fuel entering your car’s engine. The most common sensors are the Mass Air Flow sensor (MAF), the Intake Air Temperature sensor (IAT), Manifold Air Pressure sensor (MAP), the Short Term Fuel Trim (STFT), the Long Term Fuel Trim (LTFT), and the Oxygen sensor (02). A few examples will explain the interaction of these sensors and their role in engine diagnosis.

Symptoms of P0171

The lean condition should disappear at full throttle, but be noticeable at idle if there is a small vacuum leak.

Common Causes of P0171

Unmetered air entering the engine is a common cause of the P0171 trouble code.  While the air passing through the MAF sensor is metered by the car computer, intake leaks or vacuum leaks are not known.  These sources of additional air increase the air to fuel mixture.  A quick way to detect a leak is to observe the LTFT readings at idle and full throttle.  At full throttle, a small leak will contribute far less air to the large overall volume of air entering the engine.

A lean condition, which can lead to the P0171 trouble code, is much more common than a rich condition. A lean condition is detected when the O2 sensor responds to the presence of oxygen by inhibiting its voltage output. Said another way, an oxygen sensor will produce an output of 1 Volt if the exhaust mixture does not contain oxygen. A typical 02 sensor reading for a lean condition will be much lower than 1 Volt as shown below.


When the car computer detects a lean condition in the O2 sensor, it will immediately try to correct the lean condition and return to the stoichiometric ratio by adding fuel.  Enter your short term fuel trim (STFT).  The short term fuel trim is a percentage change in fuel delivered to the engine.  For a lean condition, more fuel is required, and the short term fuel trim will be positive anywhere between 0 and 50%.  The short term fuel trim increases until the oxygen sensor tells the computer that the stoichiometric ratio of 14.7 exists.  Once the engine is back in stoichiometric balance, the short term fuel trim will level out.  Now enter your long term fuel trim (LTFT).  The LTFT will be set at the STFT setting.  The LTFT takes over for the STFT (which will return to zero after the LTFT memorizes the STFT setting) to keep the engine in stoichiometric ratio.

What causes the P0171 code? Generally, if the LTFT is above 25% for two driving cycles, the computer will throw the P0171 code.


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