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P0172 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


When P0172 is detected, it means that the exhaust system is too rich in bank one. This code may be accompanied by P0175, which monitors bank two.


When the engine is operating correctly, there is a precise balance of air and gasoline being combusted. The engine has multiple sensors to monitor this balance, including the mass airflow sensor (MAF), manifold absolute pressure sensor (MAP), and various oxygen sensors.

When the system is too rich, that means there is a lack of oxygen in the exhaust (it’s “too rich” with gasoline).


Many different components can lead to this problem, including issues with the sensors inside the engine. Here are the most common culprits when you detect this code.

  • Dirty MAF Sensor – If the sensor isn’t clean, it can incorrectly detect how much air is in the exhaust system.
  • Faulty Oxygen Sensor – This unit detects the amount of oxygen in the engine, so if it’s broken, it can trigger the code.
  • Leaky Fuel Injector – In this case, the injector is putting too much gasoline into the engine, making it too rich.
  • Faulty Fuel Regulator – The regulator controls how much fuel goes into the engine, so it might be putting too much gas when it’s not working correctly.
  • A Vacuum Leak – Your engine is pressurized to ensure that it runs smoothly. If a vacuum is leaking, it can affect your car’s performance and trigger this code (among others).
  • Fault in the Coolant System – Although unlikely, your coolant system may have a bad thermostat or temperature sensor. If these components show a cold temp all the time, it can cause more gas to get into the engine than usual.
  • Worn Spark Plugs – Your plugs need to work precisely to ensure proper combustion. If they are worn, they can affect the air to gasoline ratio.


If the culprit of the code is a faulty sensor, you might not notice any issues while driving your car. However, because too much fuel can be a significant problem, an engine that’s too rich can cause black smoke to come out of the exhaust.

Another symptom might be higher-than-normal fuel consumption.


As with all OBD2 codes, the first step to diagnosing the problem is to get freeze frame data to see if there are multiple codes present. Then, clear all of the numbers and do a test drive. If it comes back, then it’s time to inspect your engine.

  • First, you’ll want to do a fuel pressure test. This will tell you if there is a leak in the system.
  • Next, using noid lights, you can inspect your fuel injector. If you don’t have noid lights, it’s a good idea to buy them for self-diagnostics and repairs.
  • If the problem isn’t your fuel system, then check your engine for any vacuum leaks. Inspect hoses for cracks and breaks, as they will be the primary source of a leak.
  • Locate your MAF and oxygen sensors. If they are dirty, cleaning them might help remove the code. Otherwise, you’ll have to do a process of elimination to determine if these sensors are faulty.
  • Check your air intake system for clogging. Clogs prevent air from getting into the engine, causing the mixture to become too rich.
  • Finally, don’t forget about the coolant system. Check your thermometers and temperature gauges to see if they are accurate.

Common mistakes

Whenever you change or fix something inside your engine, you will want to clear the code and do another test drive. This will show you if the problem is solved or if the culprit was something else. If you repair multiple pieces at once, it’s much harder to identify what really happened.

Also, many people forget about the coolant system. If nothing else seems to be faulty or broken, then this system is likely the cause.

How serious is this?

Although you can still drive with a “too rich” engine, the primary danger is to the environment. First, you won’t be able to pass most state’s smog checks. Also, if smoke is present in the exhaust, it can pump more pollution into the air.

Another thing to consider is that failing to fix a problem that causes a check engine light can prevent you from noticing additional issues. Since one light refers to so many codes, you could be neglecting something far more serious.

What repairs can fix the code?

repair manuals

Depending on the culprit, these repairs should help your fuel mixture return to normal.

  • Replace any cracked or broken hoses – fix a vacuum leak
  • Replacing any part of your fuel system, including the injector, fuel pump, or fuel regulator
  • Replacing a clogged air filter
  • Replacing a faulty coolant sensor or thermostat
  • Replacing worn spark plugs
  • Cleaning or replacing a faulty MAF or oxygen sensor

Related codes

Code P0175 refers to bank two in the engine. As we mentioned, sometimes both codes may be present simultaneously.


If you’re not sure how to check your fuel system, or you’re worried about doing any damage, we highly recommend taking your car into a professional mechanic. Also, monitoring your sensors is much easier when you have equipment that can do it for you.

P0172 Code – What Does It Mean & How To Fix It