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P0171 Severity
Identifying an OBD code is just the start of a complete diagnostic routine. Most trouble codes are caused by a sensor reporting a value that is out of range. Just replacing a sensor may not fix underlying issues. Be sure to understand the code, the part or system in your car that the code refers to, and the risks of delaying professional repair.
Proper diagnosis will save you time and money on unnecessary repairs.
Urgent codes indicate immediate action is necessary. These are urgent and significant malfunctions that can cause severe damage to the vehicle or harm the driver and passengers.
The engine control module (PCM) uses multiple sensors to determine the required amount of fuel needed to keep the engine running efficiently. If the PCM cannot achieve the desired balance between fuel and air, it will set code P0171.
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Combustion engines function best when they maintain a specific ratio of air to fuel (about 14.7 parts air to 1 part fuel) in the fuel mixture. To keep the engine running properly, the engine control module measures the oxygen content in the exhaust with oxygen sensors and makes adjustments to the mixture by injecting more or less fuel as necessary. When these adjustments become too large, a fault code is set.
When the P0171 code sets, the oxygen sensors are detecting too little oxygen in the exhaust (running "lean") and the control module is adding more fuel than normal to sustain the proper air/fuel mixture.
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Control module software needs to be updated
Vacuum leaks (intake manifold gaskets, vacuum hoses, PCV hoses, etc.)
Plugged fuel filter or weak fuel pump
Plugged or dirty fuel injectors
When a vehicle has the fault code P0171, it means that computer can no longer automatically adjust the mixture between air and fuel. Code P0171 applies to 4-cylinder engines (Bank 1) as they generally have only one bank. If you have a V6 or V8 engine you can also get a P0174 code which refers to Bank 2.
When the code says that the fuel system is "too lean," it means that the computer has been adding more and more fuel, which is called Long Term Fuel Trim. Ideally, the Long Term Fuel Trim should be close to 1 to 2 percent. When a code P0171 is set, it means that the Fuel Trim is anywhere from 15 percent to as high as 35 percent compensated. When this happens, the computer knows that there is an improper condition in the control of the Fuel System.
The first step in the diagnosis of a code P0171 is to look at a minimum of three ranges of the Long Term Fuel Trim numbers on a scanner. Check the idle reading—3000 RPM unloaded and 3000 RPM with at least 50 percent load. Then check the freeze frame information for the code to see which range(s) failed and what the operating conditions were.
"Lean" running cars and light trucks are highly polluting vehicles. Most of the NOx pollution, which is poisonous and can cause asthma, is caused by vehicles that are running too lean. A lean running car can also misfire, which puts raw fuel (HCs) into the catalytic converter which can result in internal damage, and on into the atmosphere. When you are behind a car or truck that is misfiring it makes your eyes burn. In comparison, a "rich" running engine (one that is not misfiring as a result) has no smell (CO is odorless) or you may detect a rotten egg smell, which is sulfur dioxide produced by the Catalytic Converter.
P0171 is not an Oxygen Sensor problem. Before a P0171 code is possible, the computer first ran a series of tests to validate the readings from the oxygen sensors. Since the oxygen sensors passed their readiness tests and didn't set any codes, the computer then looked to the Fuel Trim adjustment. When the computer determined the air-to-fuel mixture to be too lean, it then set the P0171 code.
Always check to make sure that there is not a PCM software update due or available. Often, as the vehicle's engine wears, the PCM's Fuel Map software inaccurately compensates for this condition. The fuel mixture grows lean and eventually, the code sets.
A vacuum leak is very common. It could be a torn PCV hose, a torn Intake Air Boot, or even a broken seal on the dipstick (the dipstick is a part of the PCV system and if it does not seal, too much unmetered air will enter the engine). Don’t rule out a sticking/leaking EGR Valve or leaking EGR or Intake Manifold Gasket. If it is a V6 or V8 engine and the code is only on one side/bank, it could be a defective Intake Manifold Gasket or cracked/leaking manifold.
An "under reporting" Mass Air Flow Sensor can be a common cause of a code P0171. Essentially, this means that the Air Flow Sensor is telling the computer that much less air is entering the engine than actually is.
Since the oxygen sensors are telling the computer that more fuel is needed, this causes confusion in the computer because the Mass Air Flow Sensor is still saying there is too little air and the Oxygen Sensor is reporting that the mixture is still too lean. The computer tried to compensate, but since resolution is impossible, it sets the code. It is important to restate that the Oxygen Sensors are accurate—the fuel mixture is too lean. In this case, the Air Flow Meter or Sensor is inaccurately reporting the real amount of air entering the engine.
There is a very effective "truth test" for any Mass Air Flow Sensor. Start the engine, let it idle, and then check the Barometric Pressure reading on the scan tool data. If the reading is about 26.5 Hg and you are close to sea level, you know that you have a defective Air Flow Meter because it is telling you that you are at about 4500 feet above sea level. (These conversion tables will help.) When the Mass Air Flow Sensor sees this Barometric reading, it adjusts its Air Density table and then "under reports" the actual amount of air entering the engine. It does this because the Barometric Pressure Sensor is actually part of the Mass Air Flow Sensor.
Sometimes the Air Flow Sensor and the sensing wire get covered with dirt, dust, or oil residue, which can also set a P0171. Cleaning the sensor might hold off problems for a while, but eventually, the MAF sensor should be replaced. Always make sure the Air Filter and its enclosure are dirt-, dust-, and oil-free. If you clean and replace the filter and its enclosure as needed, you will prevent the new MAF from failing.