How To Fix Code P0172: Quick Rescue Guide

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OBD scanner with code P0172 on screen

When you have an illuminated check engine light on your dashboard, that can be confusing, especially when you don't know what it means. Trouble codes like P0172 on a scan tool are something most people should understand. 

So what does this code mean? This article will lay out all the information you need to know about this trouble code, including its symptoms, causes, diagnoses, and fixes. Are you ready? Let's get started!

What is Code P0172?

When the Powertrain Control Module (PCM) detects a lean air-fuel mixture in the engine, code P0172 is triggered, showing the check engine light on your dashboard. This indicates that the PCM sees too much gasoline in the air-fuel mixture.

This happens when the downstream oxygen sensor has detected an insufficient amount of O2 molecules in the exhaust stream. The standard fuel mixture should only have a 14.7:1 air-to-fuel ratio. If the PCM detects a higher combination, this sets off the check engine light.

Symptoms of Code P0172

One of the most common symptoms of code P0172 is an illuminated check engine light. You may believe it is not significant, but you should not ignore it because there are other indicators related to this code issue, which are as follows:

  • Rough idling
  • Hesitation or stumbling upon acceleration
  • Engine misfires or stalls
  • Poor power from the engine
  • Strong fuel smell from exhaust

Causes of Error Code P0172

When fixing the code P0172, it is vital to diagnose all the possible reasons that triggered the warning light to pinpoint the exact issue. To help you in diagnosing, here are some of the common causes: 

  • A leaking or stuck open EGR valve 
  • A malfunctioning EGR valve 
  • Faulty O2 sensor
  • Damaged spark plugs
  • Defective fuel pressure regulator
  • Defective coolant thermostat

How to Diagnose Error Code P0172?

Before you start fixing your car, diagnosing all the possible causes should be done beforehand. This could make the process easier and help to pinpoint the exact problem. You should not skip this to avoid mistakes in fixing your car. So what do you need to do? Let's start!

Perform a Visual Inspection

Begin by checking for leaks and cylinder compression. Check the fuel lines, fuel injectors, and oil air filters for wear and tear as well. In addition, check to see if there is a damaged pipe in the intake manifold to verify there are no leaks. Tighten any loose or damaged connections that are discovered. If you cant fully tighten them, replace the part. 

Check the Regulator Pressure

After you've made sure all components in the system are in working order and that no leaks exist in any vacuum lines or hoses, inspect the fuel pressure regulator or a loose or broken fuel line. 

Tighten lines that are leaking/disconnected, then check the pressure with a test gauge. If the pressure is too low, you should replace the regulator.

Inspect the Oxygen Sensor

A faulty oxygen sensor can cause an excessively rich condition which triggers code P0172. Make sure not to skip this step as it is one of the most common causes of the problem. Inspect for harness or connector damage to the sensor. 

Listen for a raspy, rattling noise that could indicate a bad oxygen sensor before replacing it. You can also use an electronic testing device to determine if the upstream oxygen sensor is good or bad.

Check the Voltage Reading

When there's not enough voltage reaching the downstream oxygen sensor, this also triggers the error code P0172, as the PCM detects a lean condition. However, too much voltage can also make the O2 sensor malfunction, so ensure the reading is 0.2 volts or more at idle with the engine warmed up. 

Failure to check this will reduce gas mileage, and over time your catalytic converter will become damaged more quickly than usual.

Inspect the Oxygen Sensor

If you are still getting the error code P0172, then there's a good chance the downstream oxygen sensor is saturated. This means that it is likely shorted out or the heater has failed, which means replacing the faulty part would fix it.

If the oxygen sensor isn't outputting a voltage reading or no power has reached the sensor, this could entail a faulty or failed sensor. Replacing the oxygen sensor may solve the problem and clear the trouble code.

How to Fix Code P0172?

There are many ways to fix this code error; however, it depends on the exact cause. It is best to take your car to a professional mechanic to avoid unnecessary repairs. If you decide to repair yourself, here are some ways to solve the issue. 

Common Mistakes When Diagnosing Code P0172

Without verifying the actual cause of the code, most people may immediately try to fix it that may, in turn, add more issues or may end up being very costly. Here are some of the common mistakes people make when diagnosing code P0172.

  • Assuming that a clogged or dirty mass airflow sensor is causing a rich running condition when the real           cause is poor fuel economy and lack of power. 
  • Not checking if the EGR valve is leaking or stuck open. 
  • Not checking the EGR valve for malfunction.
  • Replacing the O2 sensor without confirming if it's really the cause.
  • Replacing the spark plugs without diagnosing the other possible causes.
  • Not checking the fuel pressure regulator.

P0172 Code FAQs

1. How serious is the code P0172?

The error code P0172 is a serious and potentially expensive issue as it has something to do with the oxygen sensor. If you leave the problem unaddressed, it could cause severe damage to your catalytic converter, requiring costly repairs.

2. What is the estimated cost of repairing the P0172?

The cost of the repair depends on what needs to be fixed. In general, a tune-up costs around $100+, while a more complicated system such as an oxygen sensor may cost up to $300+.

3. Can I drive with this code?

Yes, you can still drive with the code P0172; however, you may experience some of the signs listed above, so it is best to address the issue as soon as possible before it gets worst. 

4. What type of OBD scanner can I use to scan this code?

An advanced OBD2 scanner is the best you can use as it has live view data, which can help you understand the issue quickly and efficiently. 

5. Is it recommended to diagnose and repair this issue at home?

You can try diagnosing and fixing this at home; however, please remember that you must have enough knowledge about automobile repairs. So, if you are hesitant about your skills, then have your car checked by a professional mechanic instead. 


The code P0172 is something you shouldn't ignore, as this can cause more expensive repairs in the long run. Although many causes are associated with it, there are several ways to address them, and with the help of this guide, you can fix it yourself! 

We hope this article has walked you through addressing your problem with code P0172. Did we help you turn off the annoying check engine light on your dashboard? If so, you are free to share your thoughts below!