Do-it-yourself auto repair

P0171: error code “lean mixture”. The reasons and ways of fixing the error р0171

Lean mixture code P0171

The error code P0171 appears due to lean mixture, when there is lack of fuel and oversupply of air. This mixture appears when the engine acquires too much air from outer environment and/or the fuel system injects not enough fuel.

P0171 lean mixture

Such state of the mixture can cause the slips of ignition, the delays of the car reaction for accelerator movement, stumbling during acceleration, engine dips during idle run, the unstable work of the engine. In the engine control systems Euro-2 and higher the lambda-sensor controls the mixture quality, in which there’s one portion of fuel per 14,7 portions of air. Should the air exceed the normal amount, which is usually 20-30%, the error code P0171 appears.

How does the car act with the lean mixture

In case of lean mixture presence, the car engine will work unstably during idle run, will overheat due to lower of speed of combustion of this mixture, there will be popping-back in the intake manifold. The car itself may react and move into top gear slowly during load.

In case of lean mixture the working temperature of the engine rises which leads to its overheating and valves and pistons burning-out.

In case of long usage of the car with lean mixture there will be an excessive fuel expenditure due to slow moving to top gear, that’s why you will have to downshift constantly. The lean mixture appears as a consequence of misbalance of air intake and the mixing of fuel with exceeding amount of air.

The reasons of error code P0171 appearance

The reasons for exceeding air intake may be the following ones:

  • The dirt in the consumption sensor, so the amount of intake air is counted incorrectly.
  • The vacuum leak;
  • EGR valve, which is responsible for exhausting fumes recirculation, is closing insufficiently and lets the air in the intake manifold;
  • A great consumption of air via EGR valve as a consequence of EGR differential pressure transducer malfunctioning.


Should the reason of lean mixture be the lack of fuel, then the problems may be the following:

  • Insufficiently powerful oil-fuel pump;
  • Fuel filter with resistance;
  • Contaminated fuel injection nozzles;
  • Leak in the fuel pressure control;


In order to find the reason of the error P0171 it is necessary to define the reasons of the lean mixture intake to the engine and why check-engine shows such an error. Then we can make a certain plan of the car examination:

  • The nozzles and the fuel supply system;
  • Air-tightness of the intake and inlet manifold and attached pipes;
  • The detectors of oxygen and air flow (Mass airflow sensor and lambda sensor).


How to conduct the examination

Check the controllers. One of the most widespread reasons of error P0171 appearance is the contamination of the airflow detector (MAF). The speed of reaction of the detector on the change of air consumption may be a delayed one due to the contamination on its wire. The fuel fumes may contaminate is as well. They are passing through the inlet collector and the throttle body with the engine not being working. These fumes condensate on the detector wire, the wax layer of which causes the sending of the signal about the insufficient amount of the air the mixture. The engine control block also do not add more fuel with the rising of the air amount, so the misbalance appears. This may lead to lean mixture and appearance of the error P0171. The following errors as p0100 or p0102 may appear to which may mean the malfunctioning of the mass airflow detecter. In order to fix the problem one may need to clean the detector itself of clean with the help of special cleaners or with carbcleaner (spraying it on the sensitive element), or install a new detector.

For cleaning use only carblceaner of cleaners for electrodevices.

Check for vacuum leak. Should the reason be not the mass airflow detector then it is necessary to check the vacuum leak. The depressurization may occur in any place of the inlet tube and also in the end of throttle body – in the point of vacuum tubes connection of vacuum pipes, inlet collector, in the gasket of throttle body and in the gaskets of inlet collector. You should also pay attention to possible mechanical damage of the tubes of crank vent system, system of catching the fuel fumes and on the cover of the inlet collector. The whole system of exhausting fumes discharging near the airflow detector half meter before and after it should be air-tight. Otherwise the values of the detector may be incorrect (e.g., the error may appear when the corrugated tube burns out). Usually the leaks are checked with the help of car scanner (via the method of checking the fuel balancing value).

Should there be an differential pressure transducer installed in the system of exhaust gas recirculation (DPFE), then the error code P0171 may appear in case of its malfunctioning. Such transducers tend to break in case of the car long run.

It should be checked in case the run of the car is more than 90 thousand kilometers.

This detector is installed on the engine and is connected via to pipes with the tube, via which the exhaust gases are diverted to the recirculation valve (EGR). The corrosion of the detector may lead to the decreasing of the sensitivity, that’s why it may transmit the information about the deficiency of gases, so the EGR valve stays open longer, and this leads to rising of the air amount and leaning of the mixture.

Check the fuel system. As it was stated earlier, the error code P0171 may appear in case of insufficien amount of fuel, contained in the mixture. In order to check the fuel system functioning the following tests are conducted:

  1. Checking of nozzles productivity (on the stand);
  2. Checking of fuel pressure;
  3. Checking of fuel pump productivity;
  4. Checking of voltage, supplied to fuel pump;
  5. Checking of fuel pressure controller;
  6. Checking of the fuel filter state.

These tests may be performed with the help of car scanner. The tests of fuel pressure parameter and the value of fuel balance are performed.

Should the error P0171 appear periodically (not from time to time), then the reasons may be the unreliable connection of transducer harness and controller contacts, the mechanical damage of harness, the mal-grounding of the controller.