P0172 Code (Symptoms, Causes, and How to Fix)

Last Updated on June 11, 2020

Air and fuel are the heart of the internal combustion process. The proper amount of air and fuel must enter the internal combustion chamber if the ignition is going to be successful. Otherwise, you might have a situation where your engine either consumes too much fuel or under performs.

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The engine control unit (ECU) normally regulates the flow of air and fuel into the chamber. This is the central computer which manages virtually all the internal functions of a vehicle, including how much fuel gets injected into the combustion chamber.

But if the computer were to malfunction or some other problems were to exist in the fuel injection system, then it could cause the engine to receive more fuel than it needs to sustain the vehicle’s power demands. This causes the ECU to throw trouble code P0172.

What Does Code P0172 Mean?

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

Your engine control unit is constantly monitoring and communicating with the sensors of your fuel injection system. If the unit detects that too much fuel is in the combustion chamber, the powertrain will generate diagnostic trouble code (DTC) P0172.

There are several sensors and instruments used to detect the quantity of fuel within the exhaust gases of the chamber. These include the manifold absolute pressure, mass air flow sensor, and the oxygen sensors. Whenever there is too much fuel, that means there is not enough oxygen.

The term “Bank 1” refers to the area of the engine that has the first cylinder, which is technically cylinder #1. The oxygen sensors measure the quantity of oxygen in the exhaust gases that are in this cylinder.

If the sensors detect that there is very little oxygen present, it means the ratio of air-to-fuel is not where it needs to be.

The normal ratio is 14.7:1 for most gasoline engines. This ensures the engine generates the most amount of power possible with the least amount of fuel. If the engine control unit detects the bank 1 cylinder is “too rich,” it means it has too much fuel and not enough in oxygen.

Symptoms of Code P0172

The symptoms of trouble code P0172 are usually the same symptoms of an engine running rich. Normally, your check engine light will be on (unless the bulb is burnt out). But in addition to that, here are some additional symptoms:

  • Strong fuel or rotten egg smell from exhaust
  • Poor fuel economy
  • Engine hesitating
  • Engine misfiring
  • Lack of power when accelerating
  • Rough idling

Causes of Code P0172

There are several reasons why this problem could exist. The most common reasons include a faulty oxygen sensor or a dirty mass air flow sensor (MAF). In addition, here are some additional causes of throwing code PO172.

  • Faulty fuel injectors leaking fuel into the combustion chamber
  • Faulty spark plugs
  • Faulty manifold absolute pressure sensor (MAP)
  • Faulty throttle position sensor
  • Dirty or clogged air filter
  • Vacuum leak
  • Faulty ECU (or PCM module)
  • Faulty fuel pressure regulator
  • Faulty thermostat (stuck open)
  • Cooling system that’s been retrofitted (ie: thermostat removed, fan running direct, etc.)
  • Restricted or damaged fuel line

How to Fix

Because there can be so many causes of DTC P0172, diagnosing and ultimately fixing the issue can be tricky. It’s recommended to always start with the easiest options.

Check and clean your Mass Air Flow sensor and air filter first and work your way down from the easiest (and cheapest) possible causes.

It’s common for some to automatically assume they need to replace either the O2 sensor or air/fuel sensor but this is a common misdiagnosis.


20 thoughts on “P0172 Code (Symptoms, Causes, and How to Fix)”

  1. I really appreciated your explanations am experiencing all the symptoms outlined and I will try the remedies provided by you.

  2. Can rings and pistons replacement cause high pressure fuel pump to break? Does fuel pump problem cause gas smell and engine to shake when turned off?

  3. I took my car for an MOT and service (The service light indicated that it was due) and requested that the cause of the engine light be looked into. I was told that the vehicle needed an air mass flow meter so I had it changed
    My engine light remained on afterwards and my car also developed the following symptoms:
    Loss of power when continuing from traffic light stop and when going round roundabout.
    Every few days it does not start at all from rest position / key will not turn in ignition until battery is charged.
    I was told to change the battery so I changed it but it has not made any difference.
    I was told that it needs a sensor but wonder whether it might need anything else given the symptoms.

    • I have a 2003 galant. My ECL is on due to code P0172. I replace the 02 sensor upstream and cleaned the 02 downstream. But still, light on. I replace the MAF sensor with Lancer MAF also but light still coming on.. do galant and lancer have the same specs with MAF sensor ia concerned

      • I don’t know if they use the same part. I would try googling “2003 mitsubishi galant maf sensor”, grab the part number, and check to see if the same MAF sensor is used on the Lancer.

        You’re going to have to do some additional testing to figure out why it’s running rich. I would also test the downstream O2 sensor to make sure it’s functioning properly, since it wasn’t replaced.

  4. My Honda kept saying bank1 system too rich po172 so I I replaced a brand new upstream oxygen sensor and it keeps throwing the check engine light and the im readness has an x next to the 02 sensor

  5. Check engine light came on so i had it checked and code was bank 1 too rich and was told i needed to change out my fuel pump in both places on the engine. is that the case here?

    • Not sure. There’s more to a diagnosis than just reading the code, so I would defer to the guy who saw it in person. If you’re concerned, consider getting a second opinion from another shop nearby.

  6. i have a toyota premio 2010 vlvematic engine it shows the code p0172… and i have no solution i did full tuning also… stil not detect the actual problem please if any one can help please help me….

    • You’ll probably have to do a bit of diagnostic work to determine the root cause of the rich air fuel mixture. There may be a dirty sensor or an air leak somewhere, but you won’t know for sure until you do some testing. If you’re not comfortable with this, you’ll probably have to take your car to a mechanic and pay a nominal amount for the diagnosis to find the source of the problem.

  7. The most likely cause of P0172 is a sticky fuel injector. The first course of action, dump a good quality fuel cleaner in the tank and driving for 50 miles or so.

  8. I have 3 codes, 2 out of the 3 are the same that being P2178 (System too rich off idle bank 1) and the other code is P0313 (misfire detection with low fuel) – any ideas? Car seems to be running fine other than the fan coming on when parked, engine off for a short time (roughly 10 seconds), and once or twice I have smelt and eggy smell inside of the car. Many thanks.

    • At first glance I would think those are two separate issues, but I’m not sure. What year, make, and model is your vehicle?

      I suspect the egg smell is a result of the car running rich.

  9. Mines doing the exact same thing and I’ve done a tune up oil change cleaned the gas tank bought a new fuel pump and fuel filter and it is still doing the same thing. I’ve also replaced the air filter


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