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What are the functionalities of P0171 and how to repair its error?


The P0171 – System Too Lean (Bank 1) OBDII trouble code indicates that the engine’s air-fuel mixture is too lean or too much air and not enough fuel. Driving a car with the code P0171 can cause more engine damage over time because the combustion temperature rises when the engine runs lean.

When an engine runs lean, it lacks power and may stall when the car comes to a stop at a stop sign. Acceleration may also appear to be slow. At idle, an engine may run rough. If a vacuum leak causes this code, the engine may make a hissing noise. More information on vacuum leaks can be found in this article. It is also possible that a machine will be difficult to start. Another sign of a lean engine is an engine knock or detonation (pinging) under load or accelerating uphill.

What does P0171 mean?

When the engine wears out, parts break, or measuring sensors fail; the computer can make adjustments by extending the time for the fuel injectors to richen up the mixture.This code can also be followed by a P0174, which indicates that bank 2 is lean.The ideal stoichiometric ratio is 14.7 parts air to 1 part fuel, enabling a good balance of fuel economy, performance, and lower emissions during combustion

How is the code P0171 diagnosed?

Diagnosing the code P0171 is fairly simple with a good scan tool that displays live data. The only way to find the problem without a scan tool is to check the basics and research common problems with your specific car make, year, and model. We have done some research and have posted common issues for some vehicles; please continue reading. 

Your local dealer is the best place to take your car for diagnostics because their technicians are factory-trained and are familiar with the common problems with the vehicles they service.

Basic under-the-hood checks include: is the air filter box properly closed? Is there any cracking on the intake boot? Is it possible that any of the vacuum lines or hoses have become disconnected?

Check the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) readings at idle and higher RPMs if you have a scan tool (read more about fuel trims below). Both the LTFT and the STFT should normally be close to zero. 

If the STFT rises more than + 15-20% at idle but drops at higher RPMs, you most likely have a vacuum leak. If the STFT is close to zero at idle but rises to 15-20% at higher RPMs, the cause could be a faulty mass airflow sensor, clogged catalytic converter, or low fuel pressure (bad fuel pump).

What is the difference between an intake and a vacuum leak?

An air leak between the engine intake and the mass airflow sensor is commonly referred to as a ‘vacuum leak’ or ‘intake leak.’ Vacuum leaks are typically caused by ripped or cracked intake boots, leaking intake manifold gaskets, a stuck-open or defective PCV valve or pulled PCV hose, and a broken or disconnected vacuum line.

Most common causes of the code P0171.

Searching for technical service bulletins is one way to find a common problem with your make and model (TSBs). For example, Toyota Technical Service Bulletin (TSB) EG045-07 for the 2003–08 Toyota Corolla and Matrix equipped with the 1ZZ–FE engine mentions leaking intake manifold gaskets in subfreezing temperatures as a possible cause of the code P0171, among other things. If a leaking manifold gasket is discovered during the diagnostic process, it must be replaced.

What exactly is the Fuel Trim?

Short-term fuel reduction (STFT) The engine computer (PCM) begins cycling the air/fuel ratio between a little lean and a little rich after the front oxygen sensor warms up after a cold start. This cycling occurs once or twice per second. Short-term fuel trim refers to the adjustment of the air/fuel ratio (STFT). 

This cycling can be seen on the graph here: oxygen sensor. In Live Data mode, a specific scan tool displays the STFT as a percentage denoted by “-” or “+.” For example, if your car had a small vacuum leak, you would notice the STFT increasing to + 10-15% at idle and decreasing to + 3-5% at higher RPMs.

How is the code P0171 set?

Suppose the air-fuel mixture becomes too lean and the engine computer (PCM) cannot compensate by adding fuel. The engine computer (PCM) illuminates the “Check Engine” light (MIL) and assigns the trouble code P0171 – System Too Lean for Bank 1 and (or) P0174 – System Too Lean for Bank 2, depending on which cylinder bank is affected.

The PCM also stores the parameters’ freeze-frame (engine temperature, vehicle speed, fuel trim readings, and so on) at the time the code was sent. When either the short-term fuel trim (STFT) or the long-term fuel trim (LTFT) adjustment exceeds a certain value (usually +25 percent), the code P0171 is typically set.

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