P0172 – Meaning, Causes, Symptoms, & Fixes

Code P0172 Definition

System Too Rich (Bank 1)

What Does P0172 Mean?

Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. When the upstream oxygen sensor detects there are less than 14.7 parts air to 1 part fuel in the air-fuel mixture, a rich condition exists. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the rich condition by injecting less fuel to the mixture in an effort to maintain the proper 14.7:1 air-fuel ratio. When these adjustments become too large, code P0172 is triggered.

P0172 Symptoms

  • Check Engine Light is on
  • Lack of power from the engine
  • Rough idle
  • Engine hesitating
  • Engine misfiring

P0172 Causes

  • Dirty or faulty Mass Air Flow (MAF) sensor
  • Faulty oxygen sensor
  • Leaky fuel injectors allowing too much fuel into the combustion cylinders
  • Worn spark plugs
  • Stuck fuel pressure regulator
  • Faulty coolant temperature sensor
  • Faulty coolant thermostat

Code P0172 Severity – Moderate   

It is okay to drive a vehicle with P0172 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.

Code P0172 Common Diagnosis Mistakes

It is important to complete the entire diagnostic process when diagnosing P0172. Many people will replace the air fuel sensor or O2 sensor as soon as they get a bad reading, but the root cause is often a dirty or faulty Mass Air Flow (MAF) Sensor or vacuum leak, thus causing the O2 or A/F sensor to read differently to compensate. Reading and analyzing fuel trims and the freeze frame data is the key to properly diagnosing P0172.

P0172 Diagnosis

Difficulty of Diagnosis and Repair – (1-4)

  1. Use FIXD to scan your vehicle to verify P0172 is the only code present. If other codes are present, they must be addressed first.
  2. Inspect your air intake box and air intake snorkel for any obstructions that would prevent sufficient air flow in the engine. Inspect your air filter to ensure it is not dirty and it is seated properly.
  3. Remove the Mass Air Flow (MAF) Sensor and clean the sensor using mass air flow cleaner.
    1. Reinstall the Mass Air Flow (MAF) Sensor and clear the check engine light using FIXD. If the check engine light comes back on with code P0172 continue the diagnostic process.
  4. If check engine light code P0172 persists after you have inspected the air intake system and cleaned the Mass Air Flow (MAF) Sensor, perform a fuel pressure test. If any components in the fuel system are failing, replace them as necessary. Pay special attention to the fuel pressure regulator and the fuel injectors. If the fuel pressure regulator is stuck, it can cause a rich condition due to the high pressure. If the fuel injectors are faulty, they could leak fuel into the cylinders rather than delivering the precise amount needed for the air-fuel ratio.
  5. Check that your coolant temperature sensor and coolant thermostat are functioning properly (see steps 3 and 4 of P0128). If either of these are not functioning properly, the vehicle will stay in “open loop” operation and continue to deliver a fixed rich mixture.
  6. If the check engine light persists after this diagnostic process, it is most likely time to change the oxygen sensor. You can test the oxygens sensor to verify this is the fix. Here is a great video that clearly explains this process!

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Comments 10

chris jones

July 3, 2018

Awesome set up ! Kudos !!!


August 12, 2018

Mom took her car to the mechanic & he told her she had an oil leak, but couldn’t find it. I found that diagnosis strange considering the oil light wasn’t on, but the engine light was. Got FIXD and found out it was a different issue. Guess we gotta find a new mechanic. Thanks FIXD!

Tim Dyer

September 25, 2018

I knew fixd wasn’t a waste of money, thank you.

FIXD Marketing

September 25, 2018

Thanks Tim!
We appreciate it.


Mario Hernandez

October 22, 2018

Thank you fixd I saved a lot of money


October 23, 2018

02 chrysler town and country code p0172 fix….can anybody help

Lawrence Cline

November 7, 2018

On a 2006 Honda Pilot the MIL continuously throw codes p0172 and p0175 simultaneously (never one at a time) after reset roughly after 44 miles driven. All probable components were changed out with new one’s. M..A..S, Heated upstream sensors Bank’s 1 & 2 , “BOTH” not “ONE” as listed Water temp sensor’s , Spark Plugs , Air Filter, New Thermostat , Anti-freeze, Checked and verified Fuel Pressure Regulator funtioning good, Checked and Verified no leaking or debris issues at the injectors. So what should be added to the check list is the following items in terms of difficulty. .” Air Intake Tube ” mine was riddled with with cracks easy to install not too expensive . While removed clean throttle body with approved cleaner. So now I am left with another task that is not listed .Adjust the Valves ” DO NOT ATTEMPT THIS unless you have experience or are confident you can . It is around a 3-4 Hour Job . Trust the Pro’s on this. You can severely damage the engine if something goes wrong. Shop around for the best dealI will post if thit is the cause.


May 29, 2019

I have an 04 Honda Accord V6 and engine light came on. Got a diagnostic code of P0172. What is that? I was told to change the air filter. Does that sound right?

Angel Collazo

August 23, 2019

Thanks for narrowing it down, i got the code from a automotive parts store, and gave me a huge list of what parts it may be 13 in total. 2003 Ford Taurus SEL 3.O sleeper. LoL Thanks FIXED!!

denise Brooks

August 25, 2019

my 2004 Volvo s60 bank 1 p0172 how will I no which one it is everyone is asking me I don’t no

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