Error Code P0172 is described as Fuel System Too Rich, which basically means bank 1’s oxygen sensor has detected a rich condition of fuel. This means there’s not enough oxygen in the exhaust from too much fuel being delivered into the air and gas mixture for combusion. For V6, V8 and V10 engines, Bank 1 is located at the side of the engine that contains cylinder #1.


Combustion engines work by burning the mixture of air and fuel of about 14.7 to 1—14.7 parts air to 1 part fuel. If the air ratio decreases to lower than 14.7 parts, this will be considered a “rich” mixture. If the air rises above 14.7 parts, it is called a “lean” mixture. Either situation will cause combustion to become inefficient and reduce gas mileage or cause the car to not run properly.

  • A Rich Mixture means there is too much fuel and not enough air.
  • A Lean Mixture means too much air and not enough fuel.

For the engine to run properly, the ECM (Engine Control Module) works by measuring the level of oxygen content in the exhaust with the O2 sensor, making adjustments all throughout to the mixture by injecting less or more fuel to ensure the right ratio of air and fuel.

The ECM works under specific require parameters and normal conditions, and makes minor adjustments to balance the mixture of air and fuel. If there’s too large of adjustment to make, the Error Code P0172 will be triggered. The O2 sensor will detect that there is too little O2 in the exhaust and the control module is removing the maximum amount of fuel in its task to keep the right mixture of air/fuel.

As a matter of fact, this error code is very similar to P0175, in some cases and scanning tools, the vehicle may show both codes simultaneously. “Rich” running vehicles are bad for the environment and health. This pollutant can be very poisonous and contributes to air pollution.

Common Symptoms

  • Check Engine light is on
  • Performance problems such as lack of power during acceleration
  • Engine misfiring or coughing
  • Trouble when idling at stoplight or when warm

Possible Causes

Error Code P0172 can be caused by any of the following:

  • Dirty or faulty Mass Airflow Sensor (MAF). Using oiled air filters can easily cause the MAF to be dirty as dirt can easily stick to oil and accumulate in the MAF. In some vehicles, the silicone potting material that is used to protect the circuitry will leak from the MAF sensor
  • Defective MAF sensor
  • Vacuum leak
  • Engine mechanical problems such as ignition misfire
  • Fuel delivery or fuel pressure problem (restricted fuel return line)
  • Stuck open thermostat
  • Problematic coolant or Air Temperature Sensor
  • Defective Throttle Position Sensor

How to Check

When error code P0172 and P0175 appears, it means the vehicle’s computer could not automatically adjust the air/fuel mixture. This code applies to Bank 1, while the P0175 applies to Bank 2. The latter applies mainly to V6 and V8 engines, as the 4-cylinder engines only comes with one bank, though there are some exceptions and some high performance 6-cylinder engines (BMW, Lexus, etc) split the cylinders into groups of two to three, and refer to them as separate banks.

When the error code shows fuel system too rich, this means the computer pulls back more and more fuel, which will be called long term fuel trim in the long run. Ideally, this should be around 1% to 2%. When P0172 is set, the means the fuel trim is somewhere around -15% or even as low as -30% compensated. When this thing happens, the computer knows there’s a bad condition in the control of fuel system or the oxygen sensor.

To diagnose both P0172 and P0175, you need to check the scanner and look for the minimum of three ranges of the long term fuel trim. Also, check the idle reading – 3000 RPM unloaded and 3000 RPM with at least 50% load. Then, jot down the freeze frame info of the code and determine range(s) failed, and identify what are the operating conditions when it failed.

Test the MAF Sensor

To test the MAF sensor, start the engine and let it idle, and then test the Barometric Pressure reading from the scan tool. If the reading is 34.5 Hg, and you’re in sea level, then that means you have a defective Air Flow Meter, as the reading suggests you are in 4000 feet below sea level.

When the MAF sensor sees the barometric reading, it automatically adjusts air density and then “over reports” the actual amount of air that enters the engine. This is because the barometric pressure sensor is part of the MAF sensor. Sometimes, the MAF’s sensing wire can accumulate and get covered with dirt, dust and/or oil residue, which can also lead to the error code. The simple solution is to clean the sensor. But it needs to be replaced eventually.

Make sure your air filter and the enclosure is free of dirt, dust and oil. Clean or replace the enclosure as needed to keep MAF in good condition.

How to Fix

Different causes require different solutions. Here are some of the most common solutions to fix this error code:

  • Check the PCV hoses and vacuums for damages, replace components if necessary.
  • Clean the MAF sensor. Make sure you check your service manual to pin point the exact location of the MAF sensor. The best way to clean it is to remove it and clean it using a brake cleaner or electronic cleaner. Be careful in cleaning the MAF sensor, and make sure you don’t damage it. Also, make sure it is fully dried before reinstalling the sensor.
  • Check the fuel lines for cracks, pinches or leaks.
  • Inspect the fuel pressure at the fuel rail.
  • Inspect the fuel injectors for dirt. Get them professionally cleaned or replaced, or clean it yourself using a fuel injector cleaner.
  • Inspect the exhaust leak located before the first O2 sensor. This is unlikely the cause of the problem, but it is still possible.

Parting Tips

For more information about error code P0172 or related error code P0175, feel free to browse through our website and other codes.