Code P0171 - System Too Lean

A Nissan TSB for Nissan Maxima dated March 2004 mentions replacing an air/fuel sensor as a solution for the code P0171. Another Ford TSB for a number of late 90-s Ford vehicles describes a problem with MAF sensor contamination where both codes, P0171 and P0174 would appear together.
Problems with the mass air flow (MAF) sensor causing the code P0171 are fairly common in many vehicles. Sometimes the mass air flow sensor just gets dirty and gives false air flow readings causing the engine to run lean. Cleaning the air flow sensor may help temporarily; the proper repair involves replacing the faulty mass air flow sensor. A mass air flow sensor can be tested with a scan tool; read more here: mass air flow sensor.

Basic car knowledge: how the air/fuel ratio is adjusted

Fuel trim Control diagram - OBD II system
Fuel Trim Control diagram

In modern cars with the OBD-II system, the front oxygen sensor measures the amount of oxygen in the exhaust gases and provides the feedback signal to the engine computer (PCM) whether the air-fuel mixture coming into the engine is lean (too much air and too little fuel) or rich (too much fuel and too little air).
The PCM adjusts the fuel supply accordingly to keep the air/fuel ratio at the optimum level, which is 14.7/1 (14.7 parts of air to 1 part of fuel).
This process is continuous: when the oxygen sensor detects too much oxygen, the PCM assumes that the air/fuel mixture is lean and adds more fuel. When there is too little oxygen in the exhaust (air/fuel mixture is rich), the PCM reduces the amount of fuel. In technical language this adjustment is called fuel trim.

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What is the Fuel Trim?

Short term fuel trim (STFT) Once the front oxygen sensor is warmed up after a cold start, the engine computer (PCM) starts cycling the air/fuel ratio between a little lean and a little rich. This cycling happens around once or twice per second. This air/fuel ratio adjustment is called short term fuel trim (STFT). You can see this cycling on the graph here: oxygen sensor. A typical scan tool displays the STFT as a percentage with "-" or "+" in the Live Data mode. For example, if your car had a small vacuum leak, you would see the STFT going up to + 10-15 % at idle and dropping to + 3-5 % at higher rpms.

Long-term fuel trim (LTFT) There is also a long-term fuel trim (LTFT), which is a long-term compensation of the base air/fuel ratio. For example, if over time, the engine gradually develops a small vacuum leak, it makes the engine run leaner (more air and less fuel). In a long term, the engine computer (PCM) will compensate this condition by adjusting the long term fuel trim (LTFT) to add more fuel. The long-term fuel trim is also displayed as a percentage with a "-" or "+" sign in a scan tool. For example, a long-term fuel trim (LTFT) at +5% means that the PCM added some fuel, although 5% is considered within normal range.

How the code P0171 is set

If the air-fuel mixture becomes too lean and the engine computer can no longer compensate this condition by adding fuel, the engine computer (PCM) turns the "Check Engine" light (MIL) ON and sets the trouble code P0171 - System Too Lean for Bank 1 and (or) P0174 - System Too Lean Bank 2, depending on which cylinder bank is affected. The PCM also stores the freeze frame of the parameters (engine temperature, vehicle speed, fuel trim readings, etc.) at the time when the code was set. Typically the code P0171 is set when either short term fuel trim (STFT) or long term fuel trim (LTFT) adjustment exceeds a certain value (usually +25%). Read more about the freeze frame.

What does the term "Bank 1" mean?

v6 engine

The terms "Bank 1" and "Bank 2" refer to a separate group of engine cylinders. Most in-line 4-cylinder engines have only one bank: Bank 1. In V6 and V8 engines, as well as in some 4-cylinder engines, the engine is "split" into two groups of cylinders; the Bank 1 is the one that includes the cylinder number one. To know which is Bank 1 and Bank 2 in your car, you can look it up in the repair manual or google it, mentioning the year, make, model and engine size of your car.

Q: What could cause the code P0171 on 2002 Toyota Corolla?

A: In the 2002 Corolla, the code P0171 is often caused by a bad or dirty mass airflow sensor or vacuum leaks. Your mechanic can check for vacuum leaks and test the mass airflow sensor with a scan tool. If the mass airflow sensor is causing the lean condition, cleaning it may help, but usually, it will need to be replaced. There also could be some other reasons, but the air flow sensor and vacuum leaks are the most common.

Q: 2004 Ford Taurus, getting codes p0430, p0300 & p0174

A: The code P0300 means that multiple cylinders are misfiring. This very well could be caused by the code P0174 System Too Lean (Bank 2). The code p0430 Catalyst System Efficiency Below Threshold (Bank 2) means that the catalytic converter for the bank 2 doesn't work properly. This also could be a result of the same problem that caused the code P0174. We would recommend starting from the code P0174. Often this code could be caused by a vacuum leak (e.g. from intake plenum gasket, PCV hose, vacuum lines) which is not uncommon in Ford vehicles, although there are plenty of other possible reasons, like low fuel pressure, MAF, etc.; as mentioned above. There was also a Ford TSB recommending re-flashing the engine computer, we would also recommend checking if this bulletin applies to your vehicle. By the way, a catalytic converter could still be covered by the 8 years/80000 miles emission warranty, you can verify it with your local Ford dealer.

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