Code OBD2 P0171, how to check and reset, solutions

Mysterious error codes, which are displayed on the scanners’ monitors, sometimes baffle drivers. In fact, all car electronics simply fixes malfunctions, and we are left to deal with them ourselves. Error code P0171, which is produced by engines of Nissan, Mazda, Opel or Toyota, means that the mixture in the combustion chamber is too lean. Let’s figure out how to reset the error and fix the problem right now.

Deciphering of fault code P0171

P0171 code scanner

No matter what type of engine or manufacturer of the car, fault code p0171 will tell you that the mixture in the combustion chamber is too lean. This means that the proportions of gasoline and air are shifted towards the latter. The electronic control unit can calculate the malfunction according to several algorithms. Some may even be unexpected to us.

Why the system is failing

For example, when the failure code p0171 appears only on a cold engine, and disappears after warming up, it would be logical to think that additional uncontrolled air enters the power system. We can spend months overhauling the power system, flushing the injectors, even the pressure in the fuel rail can be normal.

diagram p0171
Diagram of the injector fuel system

In fact, the fuel pump, which is about to retire, but is still pumping and creating pressure, may no longer respond to ECU requests to increase pressure when the engine is warmed up. A worn-out gasoline pump motor may work in normal mode for some time, but it is no longer able to sharply raise the pressure in the ramp.

As a result, ECU perceives the pump’s refusal to sharply increase pressure to enrich the mixture, as an emergency situation, because the mixture is really poor. So we can get the error code p0171.

What does code P0171 mean, causes of occurrence

The example we gave is rather atypical and is taken from our practice. Nevertheless, such cases do occur. In most situations, Check Engine or P0171 fault occur under more trivial circumstances, but this does not make the repair and error reset easier:

  1. Clogged mass air flow sensor. This is the easiest reason why the ECU is writing error code 0171 into the memory. The sensor simply counts the amount of air incorrectly, deceiving both us and the ECU. The way out is simple – check the condition of the MAF sensor, clean it, replace it with a new one if necessary.

    maf sensor
    What is the MAF sensor?
  2. EGR valve. In Europe, they do not know about troubles with EGR valve for one reason – gasoline is good and the valve serves as long as the car itself. Our situation is different. The valve gets soot, carbon deposits, slags, which do not let it close tightly. As a result, air enters the intake manifold, which no one controls. Hence the lean mixture.

    EGR valve
  3. The same recirculation system can cause another piggyback. If the EGR pressure differential sensor is faulty, it is inevitable that excess air will enter the intake.

Lack of fuel or excessive oxygen?

The situation can be viewed from several angles. We have just started from the most obvious one – excess air in the intake. In addition to the reasons described above, too much air can be caused by depressurization of the fuel system or the intake manifold.

Nevertheless, another point should not be missed – the lack of fuel. And this can happen due to various reasons:

  1. Clogged fuel filter. In some cars, the fault P0171, for example, Honda Civic, occurs due to the fact that the system does not provide a fine gasoline filter. The mesh on the fuel module is clogged, and then the error may occur when the engine is cold, or may occur at high RPM, when the gasoline consumption increases.
  2. Excessive fuel filter. Tuning of the fuel system is not always appropriate. For example, if we decided to install an extra fuel filter, it will inevitably give increased resistance in the system, which the ECU will perceive as an emergency situation and generate a fault p0171.
  3. Clogged injectors. Everything is clear here. If at least one injector does not produce the calculated amount of fuel, the ECU will instantly calculate this and notify us of the error.
  4. Faulty fuel pressure regulator. In this case, the valve artificially cuts off the gasoline in the right quantity, and the ECU reacts to it, as it should, producing a fault p0171.
fuel system
Fuel supply system of the injector engine

Malfunction code P0171. How to check

The difficulty is that we will have to check for a long time. The computer can’t tell you the reason why the mixture is too lean, so we’ll start with the simplest test.

Checking the fuel system

First of all, we measure the pressure in the fuel rail. In most cars for this purpose there is a special fitting in the ramp, to which a pressure gauge is connected. The average pressure in the system is specified in the user’s manual of a particular car, but in general the injector engine must have pressure not less than 3 atmospheres at startup and not less than 2.5 at idle. If you squeeze or block the fuel return hose, the pressure can rise to 5-7 atmospheres.

Clogged injectors may be the cause

If the pressure is normal, the injectors are suspect. It is possible to check them only on the stand. Besides, you can check with your own hands the voltage on the electric motor of gasoline pump, the condition of the filter on the fuel module, gasoline pressure regulator.

Checking the sensors

The most common cause of the check engine and error p0171 is still low-quality gasoline. Indirectly. It clogs the air mass flow sensor, after which it is not able to perform an objective check. The way out is as follows – flush the MAF with carburetor cleaner, put it back in place and see if the error is reset.

DPFE system recirculation valve and sensor

Associated errors with clogged MAF sensor are p0100 and p0102, which indicate a malfunction of the sensor itself. The best option in this case is to replace the sensor, although a properly cleaned device will work correctly for some time.

When working with the sensor it is necessary to remember that its working organ is very sensitive to aggressive liquids, therefore for cleaning we will use only means for carburetors.


Checking the air duct tightness

You won’t have to check here for a long time. We are interested in the areas that can let air through to the intake manifold and the manifold itself. Check it carefully:

  • vacuum hoses, their fit and integrity, presence of cracks and tears;
  • inlet manifold gasket;
  • hoses of the crankcase ventilation system;
  • the exhaust duct up to the first lambda probe on the catalyst, there must not be any air leaks here as well;
  • If the mileage is nearing the 100,000 km mark, the DPFE differential pressure sensor and EGR recirculation valve are most likely to be at fault.
A vacuum brake booster can also cause depressurization

This way you can find out, eliminate and reset the error p0171 on any injector engine. Good luck to everyone and full tanks of clean gasoline!

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