P0172 – System too rich

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If your check engine light turns on, you will need to run diagnostics on it. When the code reader shows up the P0172 code, there is a problem with the engine having too much gasoline. Modern engines require a specific mixture of air and fuel, approximately 14.7:1. When the combination is out of balance, then the check engine light turns on, and P0172 is set. 

P0172 code definition

P0172 code definition (generic): Fuel system too rich

P0172 Mercedes: Fuel system 1/1 rich

P0172 Honda: Fuel metering B2 out of spec

P0172 Toyota: “System too rich (A/F rich, bank 1)”

P0172 Nissan: Fuel sys rich B1

P0172 Ford: Fuel system too rich (cylinder bank 1)

Here's where you can get a thorough understanding of the P0172 OBD2 code
If you want to learn about the P0172 OBD2 code, this is the right place

What does P0172 mean?

Modern vehicles have precise air-fuel mixture requirements. When the ratios are off, several sensors notice. In the combustion chamber, exhaust gasses trigger the mass air-flow sensor, the oxygen sensor, and the manifold absolute pressure sensor. 

The preferred ratio of air to fuel is 14.7:1. This ratio keeps engines functioning ideally with a low rate of fuel usage. The sensors in the exhaust chamber notice the levels of oxygen and carbon monoxide. When the levels are off, the air-fuel ratio is off. The Engine Control Module will adjust, but if the ratio is too far off, the P0172 code will set. It generally means that the mixture is off, as the fuel is too rich and the oxygen level is too low. 

The technical description of code P0172 is “System too rich,” which is referring to the mixture of fuel in the engine. 

In Mercedes, the P0172 code can mean two things. First, the fuel injection system is not operating as it should. Second, there is too much fuel in the air-fuel mixture. Either the check-engine light will turn on, or your fuel consumption will noticeably increase. 

In a Honda, if the powertrain control module notices that the 14.7:1 air-fuel ratio is off and it cannot adjust, the P0172 code is triggered. Other symptoms of rich fuel mixtures include a rough idle, engine hesitation or misfiring, and lack of power from the engine. 

In Toyota, the P0172 code involves an air-fuel mixture with too much fuel. The possible symptoms include the check engine light turning on and higher than normal fuel consumption. The engine control module cannot adjust the ratio, so the check engine light is triggered instead. 

The P0172 code is a commonly diagnosed problem in Nissan vehicles. The system is triggered when the engine has too much fuel compared to the air-to-fuel ratio. An OBD2 scanner diagnoses the code.  

In Ford, the P0172 code is often diagnosed along with a P0175 code. The two codes show that the air-to-fuel ratio is off, but the P0172 means that there is too much fuel. The rich mixture is in a V-style engine. Oxygen sensors in the exhaust system notice too much fuel before the exhaust enters the catalytic converter. 

What are the symptoms of the P0172 code?

The symptoms vary in different makes, but with a rich ratio of fuel to air, cars may misfire, or fuel consumption may increase. Usually, the check engine light will engage. With too much fuel in the engine, the vehicle may idle roughly, or you may notice the engine does not have typical power. The motor might hesitate, too. 

What are the causes of P0172?

There are several possible causes of a P0172 trigger. One is a leaky fuel injector that is letting too much fuel get into the cylinders. The spark plugs could be worn. The rest of the problems are related to sensors.

If the Mass Air Flow sensor is dirty or needs to be replaced, the P0172 could be set. If the oxygen sensor, coolant temperature sensor, or coolant thermostat are faulty, the code P0172 could be set. Lastly, a stuck fuel pressure regulator could also trigger the P0172 code.

The P0172 code could be triggered by a vacuum leak or an issue with fuel pressure. 

How serious is code P0172?

Code P0172 is a moderately serious code that does not need immediate attention. However, if the problem is not fixed, the engine might overheat and create damage inside of the engine. The problem might cause a car not to pass a state vehicle inspection. Some vehicles will release a back smog from the exhaust system.

How to diagnose and fix the P0172 code?

P0172 code can be diagnosed by OBD2 scanner.
Use an OBD2 scan tool to diagnose the P0172 code.

Tools needed:


When the P0172 code is given, you need to confirm and diagnose the problem. You should test the fuel pressure and check the pulse of the fuel injector using noid lights. It would be best if you also looked for vacuum leaks. Then, check two sensors, the Mass Air Flow Sensor and the Oxygen Sensors. Finally, check for clogs in the air intake system.

Once the problem is found, there are several repair choices. Some cars will need the vacuum leak repaired, while others might need a new air filter. If the problem is a malfunctioning sensor, the sensor will need to be replaced or cleaned. The thermostat or spark plugs might need to be replaced.

You might see that PCV hoses need to be fixed. The fuel lines should also be checked to see if they have any cracks or issues that cause the mixture to be too rich.

The way to fix P0172 code.

What common mistakes to avoid while diagnosing code P0172?

Some technicians do not check the coolant temperature sensor. With a cold engine, a richer mix of fuel is added to the engine to help it warm. If the coolant temperature sensor is malfunctioning, the engine computer might continue to keep the fuel mix rich because it thinks the engine is cold. 

Tips to avoid P0172 in the future

There isn’t much that a driver can do to avoid having a P0172 code cause a check-engine light to turn on. Pay attention to fuel consumption. If you burn more fuel than usual, it’s time to visit your mechanic. Use the fuel octane that your car manufacturer requires and get regular oil changes and maintenance. 

Read more: P0171 and P0174: Meaning, Symptoms, Causes, and Fixes