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P0172 OBD-II Trouble Code: System Too Rich (Bank 1)

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P0172 code definition?

P0172 is a trouble code that is set when the ECU senses that there may be too much gasoline present in the air-fuel mixture of the engine.

A proper air-fuel ratio of about 14.7:1 is vital to obtain maximum engine power and fuel economy.

What the P0172 code means

P0172 indicates that there is too much gasoline being detected in the exhaust gases exiting the combustion chamber. The ECU uses a number of instruments, such as the mass air flow sensor (MAF), oxygen sensors, and manifold absolute pressure (MAP) to monitor the air-fuel ratio of the engine.

Most routinely, the air-fuel ratio will be measured through the oxygen sensors calculating the amount of oxygen and carbon monoxide present in exhaust gases. The best air-fuel ratio modernly known to man is 14.7:1. This particular ratio was chosen because it was discovered to have the highest power output but lowest fuel consumption rate.

The ECU has the ability to slightly adjust the air-fuel ratio if it is rich. However, if the margin of compensation is too large, then a P0172 code will likely set. The term “rich” in this case means that there is too much gasoline and not enough oxygen detected in the air-fuel ratio of the engine.

What causes the P0172 code?

  • A dirty mass air flow sensor overstating the amount of air entering the engine which may cause excessive fuel delivery
  • A bad oxygen sensor
  • A leaking fuel injector allowing too much fuel to enter the combustion chamber
  • A faulty fuel regulator, which may not be regulating the pressure of the going to the engine as it should
  • A vacuum leak
  • A possible fault in the coolant system such as a stuck thermostat or bad coolant temperature sensor
  • Worn spark plugs

What are the symptoms of the P0172 code?

  • A Check Engine Light being set
  • Higher fuel consumption
  • Black smoke coming from the exhaust

How does a mechanic diagnose the P0172 code?

  • By performing a fuel pressure test
  • Using noid lights to check fuel injector pulse
  • Checking the vehicle for vacuum leaks
  • Inspecting the Mass Air Flow Sensor and Oxygen Sensors
  • Checking the Air intake system for clogging

Common mistakes when diagnosing the P0172 code

A technician may not inspect the coolant temperature sensor and cooling system. When the engine is cold, more fuel is added to help warm it up to a desirable temperature. If the coolant temperature sensor is not sending the proper signal there is a possibility that the engine ECU may assume that it is always running cold.

How serious is the P0172 code?

  • The car may not be able to pass a state vehicle inspection because of the P0172 code setting the Check Engine Light.

  • If the the air-fuel ratio is rich enough, a black smog will come out of the exhaust, which is harmful to the environment.

What repairs can fix the P0172 code?

Additional comments for consideration regarding the P0172 code

It is highly recommended to check the condition of the spark plugs and the operation of the coolant temperature sensor.

Need help with a P0172 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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