DIY Steps to Diagnose Code P0172, how to fix

Regardless of whether the car is filled with gasoline or gas, it is not the pure fuel itself that burns in the cylinders, but the air-fuel mixture, since nothing can burn without oxidation. The oxidizer is oxygen from the air. You can not cheat physics and if the Check Engine lights up, and the scanner or diagnostic computer shows the failure code P0172, there are problems with the proportions of air and fuel in the combustion chamber.

Deciphering of failure code p0172

Diagnostic code 0172 indicates an error only if there is only one error, all sensors are OK and no other errors are detected

Before checking the engine with the scan tool, make sure there are no other errors related to the oxygen sensor, crankshaft position sensor, throttle position sensor and crankshaft position sensor. Only in this case the error code detection can be considered correct.
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In addition, a certain ratio of mixture components is required for optimum engine performance – 15 kilograms of air to 1 kilogram of gasoline. If there is a shift in favor of more air, the mixture becomes lean. In the opposite situation, when there is not enough air, the fuel mixture is too rich.
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At a certain point in the operation of the vehicle, the display may show Error code P0172 or system too rich, an error code indicating an over-enriched mixture. It can be either due to lack of air (which happens more often) or in case of too much gasoline.

Symptoms of the failure code P0172

Nothing under the hood goes unnoticed. Therefore, if you are careful, you can figure out the problem without a scanner

In addition to the error message, this fault shows itself in these ways:

  • Severe fuel overrun.
  • Typical popping in the muffler at high revs. Which are caused by the fact that the overcharged fuel can not be burnt out completely in the engine and gets into the exhaust system, there it settles and, having heated up to the required temperature, explodes. Eventually, it can cause the muffler to burst. In addition, the exhaust smoke from the muffler turns black, due to the fact that the fuel exploding in it does not go through any filtration.
  • Reduced engine power. The enriched fuel-air mixture does not burn completely, so the piston does not get enough force and the engine power decreases.
  • Filled spark plugs. This symptom is detected by inspection. Remains of unburned gasoline flood the plugs, disrupting the spark formation process.

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Diagnostics and fixing P0172. How to check and reset the code

Simple diagnostic procedures, which you can do yourself, will help to reset the error and fix the problem

It all depends on what caused it. Self-diagnosis will take at most an hour and a half of time, after which the fault code can be reset. 

There may be different reasons for this malfunction. Once you understand the cause, you can easily eliminate the error. The first step in diagnosing a vehicle is simple:

  • Check the air filter. The filter is clogged, which is the reason of low air intake. The problem is solved by filter replacement. If it is clean, go to the next point.

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  • Inspection of the injectors, whose malfunction causes an excessive amount of fuel to be injected into the operating system. Inspection of injectors is much more labor-intensive. Dismantle the fuel frame, fix the power supply socket, fuel pipes and the negative terminal of the battery. Under each nozzle we put glasses of the same volume.
    Start the engine, when the injectors work, it becomes clearly visible which one of them is faulty. In this case, the faulty injector is replaced or washed. To clean the injectors, buy a carburetor cleaner with a tube, connect the injectors to the battery, insert the tube from the cleaner inside. The injector flushing has a temporary effect. Replacement will prove better.
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  • Inspecting the injectors, check the pressure in the fuel frame, as too high values may cause an error. If the pressure is higher than the set norm, check the pressure regulator.
  • Another reason for the error may be a broken mass airflow sensor. First, inspect the MAF sensor visually for damage. Then we proceed to check the contact. To diagnose the performance of this part, you need to buy a multimeter. Connect the wires of the device to the sensor contacts, turn the ignition. The result of the test will be shown on the multimeter display.
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  • Inspect the oxygen sensor (wires may be broken).
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  • Check the spark plugs and coils.

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  • The next cause of mixture enrichment may be the ingress of excessive amounts of gasoline into the lubrication system.
  • Finally, a missing or erratic contact on one of the terminal blocks can also cause a P0172 fault. Each one must be checked individually.


After the cause of the fault has been identified and corrected, you must erase the fault code from the memory to reset the long term correction. After that, start the car and let it run at idle speed. Finally, perform a diagnostic drive.

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