P0172 Fault Code
You can find information about the P0172 fault code on this page.
It is one of the most popular fault codes that people search for on this website (in the top 50).
We have given it a seriousness of 5/10 with 1 being the least serious and 10 being the worst.
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Fault code P0172 means that the engine control module (ECM) has detected that the first bank of the engine is running too rich.
Running rich is a term used to describe when there is too much fuel or too little air in the air-fuel mixture at any given time. The ideal air-fuel ratio is 14.7 parts of air to 1 part of fuel.
If the powertrain control module (PCM) fails to revert the air-fuel mixture back to the desired levels through the manipulation of fuel injectors and intake valves, this code will be logged.
Conversely, P0171 will be logged if there is too little fuel. This represents the engine is running too lean.
This error also informs which side of the engine is suffering the issue. In this case, the first bank is at fault. Also referred to as bank “A” and bank “B.” These can be found in your owner’s manual.
There are only a few manufacturers that suffer from engines running too rich as it can be relatively simple for the PCM to rectify. These include Subaru, Dodge, and Buick.
Other Signs of This Problem
The signs of an engine running too rich are more subtle than those demonstrated with an engine that is running lean. However, they can be noticed depending on how rich the mixture is.
The first and most noticeable is the presence of the Check Engine light.
Another symptom that can be easily noticed in a severely rich engine is the presence of black smoke from the exhaust. This occurs as a result of unburnt fuel leaving the exhaust system.
Another telltale sign is a noticeably higher fuel consumption as the engine is essentially throwing away extra fuel to operate as it would normally.
If your vehicle is demonstrating these symptoms without the presence of the P0172 code, look for the mirror code P0175. This represents the same issue but in bank 2 of the engine.
What Could Be The Problem?
The main causes of an engine running too rich are the fuel, heat, and air systems. However, the fuel system is of particular concern.
A dirty or faulty mass airflow (MAF) sensor could be feeding the PCM incorrect readings. This would result in an overcompensation from the fuel injectors. This could also come from a faulty oxygen sensor.
Damaged or faulty fuel injectors can also cause this issue as they deliver excessive fuel. A faulty fuel regulator would do the same.
Other mechanical issues like a vacuum leak or worn spark plugs could also play a role.
(You can find 2 DIY methods to fix the P0172 fault code taught to you visually in this YouTube video)
The fixes for this type of issue are relatively straightforward. The systems that provide head, fuel, and air all need to be checked for issues.
The MAF and oxygen sensors can easily be cleaned or replaced if they are damaged or faulty. Cleaning or replacing any fouled spark plugs can also assist in the issue.
If there is any issue with the fuel injector in question, it must be replaced or sent to an expert for repairs. The fuel pump and fuel regulators should be checked as well and replaced if faulty.
Fault code P0172 can be considered a low to moderate concern. It does not represent as much risk of damage to the engine as running too lean.
It should be fixed within a few weeks ideally.
However, the vehicle will likely suffer significantly increased fuel economy, as well as potentially pollute the environment through black exhaust smoke.
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