OBD2 Code P0172: What It Means

Figuring out why your Check Engine Light is on means deciphering OBD-II (or “OBD2”) codes, and that can be kind … Continued

Figuring out why your Check Engine Light is on means deciphering OBD-II (or “OBD2”) codes, and that can be kind of annoying. That’s why Car Bibles did it for you! You’re welcome, now let’s get into solving your issue.

OBD2 P0172: What It Means

This code reads as system too rich (bank 1)” which means there is more fuel, or less oxygen, than is supposed to be being burned in the engine. This should be taken reasonably seriously, but running rich is better than running lean. A car burning fuel is less likely to damage itself than one burning not enough, everything else being equal. But we’re here to determine if you have a serious problem.

Likely Symptoms 

Here’s what you may experience due to the P0172 code:

  • Check Engine Light.
  • Fuel smell from exhaust. If the engine is running rich enough, or bypassing enough raw fuel, you can usually smell raw gas or more noticeable emissions from the exhaust.
  • Rough engine running. Too much fuel can flood a cylinder and cause some bad running, or the engine running sub-optimally will cause roughness.
  • Decreased fuel economy. Since there’s extra fuel being burned, consumption will naturally increase.
  • Decreased power. The engine running outside of its optimum will cause a loss of power, especially with a rich condition.

Probable Causes

Here’s what could be causing the issue:

  • MAF sensor. The job of the Mass Air Flow (MAF) sensor is to read incoming air that’s sucked into the engine. If it doesn’t read air correctly, the engine can add extra fuel for air that doesn’t exist. A faulty MAF will cause rough running and bad fuel economy, and a potential rich condition.
  • Bad oxygen (O2) sensor. The oxygen sensor is responsible for monitoring how the engine is running, if it is faulty, it could bias the engine to run rich or lean. Inspect the sensor, most OBD scanners can provide a voltage reading for easy diagnosis.
  • Bad/clogged fuel injector. In rare cases, a stuck injector can cause a genuine rich condition by feeding too much fuel into the engine. This is usually associated with black smoke, so be wary of this diagnosis.
  • Bad spark plugs. Uncommon, but can happen. Usually bad plugs will cause an associated misfire code. In certain conditions, bad plugs won’t fully combust the fuel in the engine, causing it to run “rich,” where the oxygen sensor will interpret the mixture as rich, even with the same amount of fuel being injected.
  • Coolant temperature sensor. The engine computer relies on the coolant temperature sensor to send the correct amount of fuel to the engine in cold and warm conditions. If it is faulty, it could send the incorrect information to the engine computer and cause a rich condition.

What Part Is Potentially Affected?

At worst, you can fry your catalytic converter. Usually engine damage is avoided, but the rough running can break engine mounts, or cause internal damage if severe enough. Depending on severity, fix as soon as possible.

Possible Fixes

Here are the most common fixes to remedy the P0172 code:

  • Clean or replace MAF sensor. The most common cause is a dirty or malfunctioning MAF sensor. Clean with parts-store MAF cleaner and see if it cures the problem. Just make sure you use a true MAF cleaner, not throttle body cleaner or some other cleaner, because this part is very sensitive. If cleaning doesn’t help, attempt to borrow a scanner and monitor voltage to see if it is functioning correctly. If not, replace.
  • Replace oxygen (O2) sensor. The oxygen sensor can be faulty and misinterpreting information from the exhaust. Usually, diagnostic tools will display live voltage from the sensor, with normal ranges displayed. If the sensor seems stuck in a voltage, or stuck high, then replace.
  • Replace coolant temperature sensor. If the coolant temperature sensor is malfunctioning, it will cause the engine computer to send the incorrect amount of fuel to the engine. If you notice the gauge being stuck or acting unusually, it is a likely culprit. Replace, and see if that works.

Finding The Parts You Need

Now that you’ve figured out what’s wrong with your hooptie, let’s talk about where you’re gonna find that part’s replacement.

There are plenty of places you can buy auto parts from, but Car Bibles gets paid if you click this Advance Auto link so that’s the one we’re serving up. Advance Auto Parts also has delivery, curbside pickup, and a host of helpful diagrams to aid your repair!

Chris Rosales
Chris Rosales

Chris has owned 12 cars of questionable quality, is an experienced motorsports photographer, and a good all-around wrench. When he isn’t tinkering with his car in his home garage, you can catch Chris in the canyons around SoCal. He also hopelessly hankers for Euros, but he honestly knows he should get something Japanese, eventually. Contact the author here.