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P0172: Engine Running Rich - Causes, Symptoms & How to Fix

By: The Moo News Network

Fuel injected engines use computers and sensors to constantly adjust the ratio of air to fuel entering the engine. This AFR (air-fuel-ratio) varies depending on load and driving conditions. When accelerating, AFR should be around 12:1. When cruising on the highway, it should be closer to 22:1. The engine can run leaner and use less gas once it gets up to speed, but it needs more fuel to accelerate. The ECM (electronic control module) is responsible for processing sensor data and telling the fuel injectors how much fuel to send into the engine at any given time.

Sensors regulate air/fuel ratio

The MAF (mass airflow sensor) is located in the air intake, and it measures how much air is going into the engine. The oxygen sensor, usually located in the exhaust manifold before the catalytic converter, measures how much oxygen is left over after combustion. With that information, the car knows how much fuel is being burned in combustion. If there is too much oxygen left over, it means the combustion process was inefficient, and fuel is going through the engine but not being used. Ideally, every bit of fuel that goes into the engine should be burned to release energy. If it's not being burned, it's being wasted, which is bad for fuel economy.

Causes of P0172

Anything that can alter the amount of air or or fuel going into the engine or confuse the computer can cause a P0172. This includes but is not limited to:

Dirty MAF sensor - The sensor is overestimating how much air is coming into the engine, resulting in the computer injecting too much fuel.
Bad spark plugs - The fuel will not burn completely, and excess oxygen will be left over in the exhaust.
Holes in the air intake - The air intake must be sealed after the mass airflow so that no air is coming in that didn't flow past the MAF sensor. In this case, the MAF will lose accuracy, the computer will be fed inaccurate information, resulting in an incorrect AFR.
Faulty fuel regulator - The computer thinks the engine isn't getting enough fuel, so it overcompensates.
Leaking fuel injectors - The injectors are spewing leaking excess fuel into the cylinders.

A common mistake when diagnosing P0172

When an engine just starts and is cold, the engine temperature sensor will tell the computer to send more fuel into the engine to warm it up faster. If that sensor is bad, the computer will think the engine is cold all the time, and it will constantly send too much fuel into the engine, resulting in a rich fuel/air mixture.

How to Fix P0172

Because this code can be triggered by the failure of many different parts, it's important to have a repair shop determine which part is bad. Sometimes mechanics will just start replacing things like the oxygen sensor, or the MAF sensor, or the spark plugs and see if that fixes it. This can get expensive. A better approach is to inspect and test each part. Mechanics may do the following.

Perform a fuel pressure test - This can determine whether the fuel pump needs replacement.
Checking the fuel injectors - If they are clogged, that could be the problem.
Check for vacuum leaks - If air is leaking into the intake and getting past the MAF sensor, that can throw this code.
Inspect the oxygen sensor and MAF sensor - sometimes a wire is unplugged or physical damage to the sensor.
Check the air filter - A clogged air filter can cause a P0172 because not enough air is flowing in, the computer is aware of it, but it cannot compensate enough.

How much does it cost to fix?

The repair cost can vary from several hundred dollars to over a thousand dollars, depending on which part needs replacing. A bad fuel pump can cost upward of $1500 to replace - most fuel pumps are located inside the fuel tank and require more labor. Spark plugs are cheap, but labor costs can be high because they are sometimes buried under lots of other parts. Those other parts need to be removed to access the spark plugs.

Is it okay to drive with P0172?

Most vehicles with this code are drivable; it depends on the severity of the issue and how badly the vehicle is running. This code can make a vehicle not drivable or it can drive just fine if the fuel mixture is just a slightly out of range. If you car is sputtering, failing to accelerate, or exhibiting any serious drivability problems, it's best to take it to a shop immediately.

Extended driving with a rich fuel to air ratio can prematurely fowl the spark plugs, cause soot accumulation on the valve crowns, make the engine oil dirtier faster, and cause carbon buildup in the exhaust. The engine will run 'dirtier' with the P0172, and this will cause other parts to fail faster.

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