OBD-II Trouble Codes

P0172 Code: Fuel System Too Rich (Bank 1)

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Is your Check Engine Light illuminated? Are you experiencing engine misfires? Or perhaps you’re noticing an unusual lack of engine power when you drive? While these symptoms are not exclusive to a P0172, they are definitely some of the more common ones associated with this OBD-II trouble code.

close up shot of a car's combustion engine
Ideally, combustion engines should burn a fuel mixture of 14.7:1.

What Does the P0172 Code Mean?

Diagnostic trouble code (DTC) P0172 stands for “Fuel System Too Rich (Bank 1)”. Your car’s computer will set the code when it determines that there’s too much fuel (or too little oxygen) in the engine’s air-fuel mixture. The bank 1 portion of the code indicates that the problem is concentrated on the side of the engine that includes the #1 cylinder (only applies to engines with a ‘V’ or flat configuration).   

Ideally, combustion engines should burn a fuel mixture of 14.7:1. This means there should be 14.7 parts air and 1 part fuel in the mixture. A drop in the supply of air would result in a rich mixture while an increase would make it lean. The computer determines the air-fuel ratio by monitoring the upstream oxygen sensor/s. 

But before anything else, anyone who wants to understand this code should first know what fuel trim is and why it matters. 

What is Fuel Trim?

Fuel trim is the fuel mixture adjustment that your car’s primary computer, which is often referred to as the powertrain control module (PCM), makes to balance out the supply of fuel and air. If you check your vehicle with a scan tool, the fuel trim is usually displayed as a percentage. 

In order for the PCM to determine if the mixture is rich or lean, the oxygen sensor must accurately read your engine’s fuel trim.

There are two types of fuel trim values in your car: short term and long term. Short-Term Fuel Trim or STFT is the value by which your engine is currently operating. It can suddenly fluctuate depending on factors like load and speed. 

Long-Term Fuel Trim (LTFT), on the other hand, is based on your car’s average fuel mixture over a predetermined period of time.

The proper LTFT values should read around 0 percent and can go up to 8 percent. Any values beyond 10 or negative 10 means there’s a problem. When the LTFT drops between negative 20 and 25, code P0172 and/or P0175 will likely be set.

What are the Possible Causes of the P0172 Code?

Like any other OBD-II code, a P0172 can be caused by a variety of problems. In other words, there is no easy way to pinpoint what’s causing the problem other than examining the critical parts responsible for maintaining the right fuel mixture. 

Here are the different possible reasons why you’re getting the engine code P0172:

mechanic holding a dirty air filter
A dirty air filter can cause code P0172.

What are the Common Symptoms of the P0172 Code?

A rich fuel mixture can cause your engine to consume more fuel. Apart from that, the underlying issues could cause damage to your engine. 

Your exhaust system is also at risk as unburnt fuel reaching the exhaust lines can combust and damage parts like the catalytic converter. 

To avoid this kind of headache, look out for these symptoms:

How to Diagnose the P0172 Code

As outlined above, there are many potential causes for the OBD-II code P0172. Therefore, diagnosing the exact issue that is triggering the trouble code can be difficult.

For an idea of how to troubleshoot the code, check out the video below:

How to Fix the P0172 Code

There are many ways to fix a code P0172, but it would depend on the exact cause. You may end up simply needing to clean a component, but in some cases, you would likely need to replace something. Therefore, there isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately, as outlined above, then perform any necessary repairs. 

And of course, remember that all vehicles are different. When troubleshooting and repairing OBD-II trouble codes, make sure to consult the factory repair information for your application. 

Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.

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