|Trouble Code||Fault Location||Probable 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.
BAT Team Discussions for P0171
- 2001 ford windstar DTC P0171
To solve my problem (P0171) the manual references Technical Service Bulletin TSB-03-16-1 or TSB-1-20-5 Does anybody know the contents of these TSB's? Thanks, Bob...
- cold starts chevy k2500hd
Hello, I am the original owner of my k2500 HD. I'm an auto body and frame tech, so I'm somewhat knowledgable mechanically, and maintain my own vehicles. Truck is as follows 6.0 engine, 4L80 e trans, 4.10 rr axle. Mods are a K&N fipk2 installed at 25,000 mi, throttle body spacer installed at 25,...
- 01 Chevy Prizm hesitates when engine cold
2001 Chevrolet Prizm sometimes hesitates on acceleration when engine is cold. Peculiarly, it only seems to do this during warm weather (50+). We've taken it in twice to a local repair shop. The first time they checked the fuel, ignition and computer control systems and replaced the mass air fl...
- 2004 Taurus DOHC 3.0 V6 Acceleration Stumble/Hesitation
The only really common thing we see on fords in the shop are the vacuum lines to the pcv mainly get all mushy and gooey. They leak vacuum and cause rough idling and in more extreme cases stalling. the check engine light does come on eventually and the code is p0174 or p0171 or both. We have not had ...
- 96 Ford Aerostar Noise/Vibration
Thank you for your response to my question at the BatAuto website about a noise/vibration in my 96 Ford Aerostar. I did some more checking based on your initial response and found the following information. The new front tires have a diameter of 26 3/8". The rear tires, which have been on the van...