OBDII Trouble Code P0172

OBDII Trouble Code P0172


P0172 – System too Rich (Bank 1)

How Serious Is Code P0172?

Stop Driving

  • If the code has been displayed for a long time
  • Your car is starting to misfire

Although it’s safe to drive with P0172 for a short time without affecting drivability, if you do so for too long you can cause internal engine damage and a failure of the car’s catalytic converter.

Related Sensors

  • Mass Air Flow Sensor (MAF)
  • Oxygen Sensors
  • Manifold Absolute Pressure Sensor (MAP)


  • Strong rotten egg smell from exhaust
  • Weak acceleration
  • Rough idling
  • A hesitating or misfiring engine
  • Unusual odors or noises

What Does System Too Rich (Bank 1) Mean?

System Too Rich means there is too much fuel in the combustion chamber in the engine. Bank 1 refers to the fact that it is the first cylinder that is affected.

The ideal ratio of air to fuel in a combustion engine is 14.7:1. There are sensors throughout the engine that monitor this ratio, including the manifold air pressure, mass airflow, and oxygen upstream sensors.

They send information to the powertrain module (PCM), which is responsible for making constant adjustments to maintain this ratio. The PCM produces the code when it finds that it is compensating too much and it can’t maintain the proper ratio, and that there is either too little air or too much fuel in the system, causing it to be too “rich.”


OBDII error code 0172 can have a variety of causes, which makes it fairly tough to diagnose. The issue is most often with the sensors themselves, rather than a specific fault that is causing too much fuel to enter the system. If it’s a sensor problem, it’s usually an easy fix. Here are the most common issues that are responsible for this code appearing:

  • The MAF sensor is faulty, clogged, or broken
  • The oxygen sensor is faulty, clogged, or broken
  • The MAP is faulty or broken

Here are some more serious issues that could be causing this code.

  • The throttle position sensor is faulty or broken
  • The fuel injectors are leaking
  • Worn spark plugs
  • The PCM is faulty
  • Faulty or stuck fuel regulator
  • Faulty coolant temperature sensor
  • The coolant thermostat is stuck open
  • Damaged fuel line
  • Vacuum leak


It is more likely that this code is caused by faulty airflow systems and the sensors that govern them than it is by a fundamental problem with the fuel delivery system. Therefore, when diagnosing and fixing this issue, it’s best to start by checking some of these systems first.

1. Inspect the PCV, airflow intake and vacuum houses for any dirt, obstructions or clogging. If they look like they need cleaning or replacing, then do that first.

2. Remove the mass airflow sensor and clean it with a brake or electronics cleaner. The service manual that comes with your car will help you locate it.

It costs about $20 to ask a mechanic to check the air intake for you and roughly $100 to get them to clean or replace the MAF.

Once you have done these steps, check if the code is still displayed. If it is, there may be a more serious problem.

3. Inspect the fuel lines for any obvious cracks, leaks, or pinches. If there are any, they will need to be replaced immediately.

4. Perform a fuel pressure test to see if any of the components are failing, including the regulator and the injectors. If they are, you will need to get them replaced.

5. Check the coolant temperature sensor and thermostat are working correctly (see P0128) as they can induce the engine to let too much fuel into the system.The cost to replace the fuel regulator or injectors can run anywhere up to $500 – depending on your service provider.

The cost of the coolant temperature sensor ($200) and thermostat ($300) being broken is similar if both components are faulty.

6. If all three tests offer no answers, then it’s likely you will need to replace the upstream oxygen sensor or the air-fuel sensor.

The cost to replace these sensors will be around $300, depending on your provider.

P0172 isn’t a code that means you have to stop driving immediately, and often it is a simple case of cleaning either the MAF sensor or your air uptake systems. However, it can be indicative of a larger problem, and if the code persists, you should not drive until you have found and solved the problem.

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