P0172 Code: System Too Rich Bank 1 (Symptoms, Causes, and Fixes)

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.

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What Does Code P0172 Mean?

OBD-II Trouble Code P0172 DescriptionSystem 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.

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