P0172 and P0175 Rich Trouble Codes

What are P0172 and P0175 code definitions?

P0172 - System Too Rich Bank 1
P0175 - System Too Rich Bank 2

What Does P0172 and P0175 Code Mean?

P0172 and P0175 are generic trouble codes that set when the engine's air/fuel mixture becomes too rich for the Engine Control Module (ECM) to correct within its set operating range. A Rich mixture is a mixture that has too much fuel or not enough oxygen. The ECM sets a target air/fuel ratio and monitors it using oxygen sensors and on newer vehicles air/fuel ratio sensors. When the ECM sees the engine running rich, it will command the fuel injectors to provide less fuel and monitor the change. If the fuel reduction goes past the ECMs predetermined range, the ECM will set a rich DTC P0172, P0175, or both.

P0172 and P0175 Symptoms

  • Check engine light
  • Poor idle
  • Excessive fuel smell from the exhaust
  • Black smoke
  • Low power
  • Engine misfire
  • Stalling
  • Hesitation
  • Hard start
  • No start
    Tip : Vehicles can experience one or all these symptoms depending on severity of the condition

What Causes P0172 and P0175 to Set?

  • Malfunctioning oxygen (O2) or air fuel ratio (A/F) sensor: The ECM uses O2 and A/F sensors to monitor the exhaust gas, this provides feedback on the air/fuel mixture delivered to the engine. If the sensor is faulty and the signal becomes lazy or skewed, it can provide an inaccurate rich signal to the ECM. This can lead the ECM to incorrectly determining that the engine is running rich and set a P0172 or P0175 DTC.

    Tip : A rich DTC due to a skewed O2 sensor may cause the engine to run lean due to the ECM taking fuel away during fuel correction
  • Incorrect air metering, Faulty Mass Air Flow (MAF) sensor: If the air entering the engine is not calculated accurately, the ECM can overcompensate and add more fuel than required. This will affect the air/fuel mixture and create a rich condition. The leading cause for incorrect air metering is a malfunctioning MAF sensor.

  • Inaccurate temperature (temp) sensor signals: The ECM uses readings from engine coolant temp and air temp sensors to adjust the air/fuel mixture. Cold air has a greater density or weight than hot air, so more fuel is added the colder the sensors read. If a sensor is inaccurate, it will cause the ECM to improperly adjust the air/fuel mixture, possibly resulting in a rich condition and DTCs.

    Tip : Most coolant temp and air temp sensors will default to it's coldest reading when open circuits are present, or they are left unplugged
  • Faulty fuel injectors or fuel pressure regulator: Fuel injectors can stick open and cause excess fuel to leak into cylinders resulting in those cylinders running rich. Fuel pressure regulators can also leak allowing fuel to enter the intake manifold resulting in a rich condition. In addition to leaking, a faulty fuel pressure regulator can also cause high fuel pressure, resulting in the ECM over-fueling the engine.

How to diagnose P0172 and P0175 DTCs?

Recommended tools

Diagnostic checks

Tip : See if any other DTCs are stored, additional trouble codes may help determine root cause of P0172, P0175 DTCs
  • Check oxygen sensors and/or air fuel ratio sensors: Using a scan tool, monitor the signal voltage of the O2 sensors and/or A/F sensors and make sure they are operating correctly. If a scan tool is not available a multimeter can be used to read signal voltage. Every vehicle operates differently so you should consult your vehicles service information to obtain the correct procedure in checking these signals.

  • Check Mass Air Flow sensor: Remove the MAF sensor and look for signs of contamination. Debris that makes it past the air filter or oil from an aftermarket air filter can stick on MAF sensor and cause inaccurate signals. If dirty, clean the sensor using a quality MAF cleaner. Using a scan tool or multimeter check the MAF sensor signal reading and compare to vehicles listed specification.

    Tip : Generally, a MAF reading of 1 gram per second (G/S) per engine liter at idle is acceptable. A scan tool is required for reading G/S. Example: a 4.0-liter engine at idle would read approximately 4 grams per second
  • Test temperature sensors for accuracy: With the engine cold (preferably after sitting overnight) use a scan tool or a multimeter to monitor the readings from the coolant temp sensor and air temp sensor. Compare that with the outside air temperature using a thermometer, all three readings should be close to the same temperature. If using a multimeter, you will need to consult service information for signal voltage to temperature conversion. Next, start the engine and watch the coolant temp reading, it should slowly raise as the vehicle heats up to normal operating temperature.

  • Check fuel injectors and fuel pressure regulator: Check that the vehicle is running at the proper fuel pressure. Make sure to follow factory information for correct specifications and procedures. Next, check that the fuel system holds fuel pressure after the engine is shut off. If the fuel pressure drops rapidly, it may be caused by a leaking fuel injector or pressure regulator. Some engine disassembly may be required to pin-point a leaking injector or regulator.

  • Inspect or replace engine oil: It is common for a rich running engine to contaminate the engine oil with gasoline. This can cause the engine to run rich even after a repair has been made. If you suspect the engine oil has fuel in it, change oil and retest system.

How do I repair a P0172 or P0175 DTC?

Due to the number of components that are involved in the operation of your vehicles engine management system. Proper and accurate diagnosis are required to pin-point the cause of P0172 and P0175 diagnostic trouble codes. As listed above there are several systems involved that could potentially be malfunctioning, so guessing could become costly.

For more information, questions, or help diagnosing your vehicle, contact the GoTech technical support team via phone or chat below.

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