P0172 Diagnostics Trouble Code: Meaning, Causes, Symptoms, and Fixes

A car engine is a fine-tuned machine that comprises several system components and car sensors. These components work in harmony and at a given ratio for optimal engine performance.

Air and fuel are the heart of the internal combustion process because, without the mixture of these two, the combustion process would not take place. At the same time, the engine needs the correct air-fuel mixture ratio for excellent performance.

Auto manufacturers designate several system components to regulate or monitor the air-fuel mixture in the engine. And the powertrain control module (PCM) oversees these systems and sensors.

If any of these regulatory systems or the car computer fails, it will cause the system to inject too much fuel into the combustion chambers. When this happens on bank 1, the car computer will register P0172 and project a check engine light on the dashboard.

This article will discuss the causes of P0172, symptoms, diagnosis mistakes, and how to diagnose and fix the problem. And we will also look at the meaning of P0172 on BMW and other vehicle models in the course of the main discussion.

p0172 code how to fix

What does error code p0172 mean?

The car computer, the brain of the car, constantly monitors, communicates, and regulates car sensors and system components of the airflow and fuel injection systems. For example, when the car computer detects too much fuel or too little air in the exhaust gases exiting the tailpipe, it will throw P0172.

The car computer uses readings from several instruments like the manifold absolute pressure (MAP), mass airflow (MAF), and oxygen sensors to monitor the air-fuel mixture ratio in the engine.

Most often, the oxygen sensors measure the air-fuel ratio in the engine by measuring the amount of carbon monoxide and oxygen present in the exhaust gases when exiting the tailpipe. A perfect air-fuel ratio in vehicles is 14.7:1. Manufacturers choose this ratio because studies have proved that it offers the best power output and low fuel consumption.

It’s important to note that the car computer is not a dummy that collects data from several sensors and projects it for the driver to see. Instead, it collects these data and works with them when necessary.

For instance, if you have a Honda, the car computer can slightly adjust the air-fuel mixture ratio. However, if the ratio offset is too much, it’ll throw P0172 on Honda. Of course, the same applies to all internal combustion engines.

The term ‘Rich’ means too much fuel or too little air in the air-fuel ratio. In any case, if your powertrain control module throws P0171, the car computer has detected too little fuel or too much air in the system.

What is the symptom of error code p0172?

Like other system issues, a P0172 will display a few issues that indicate your engine is running rich. First, the powertrain control module will display a check engine light on the dashboard. Here are the symptoms you will notice when a car runs rich.

Check engine light

Your car computer triggers the check engine light whenever it detects a malfunction in any system unit. The P0172 on Mercedes and other vehicles are no exception. The presence of the check engine light is to notify the driver of imminent issues in the system.

Bad gas mileage

The P0172 means too much fuel is entering the combustion chamber. So, one of the obvious symptoms is a high fuel consumption rate. This means constant visits to gas stations and extra bucks.

Rough idling

Auto manufacturers design engines to take a balanced ratio of air-fuel mixtures. Therefore, if there is a high volume of air or fuel, you’ll experience rough idling.

Engine hesitation

Another symptom of P0172 issues is engine hesitation, especially when towing, climbing uphill, or cruising on a level road. You may feel like the engine is hesitating.

Engine misfiring

A P0171 on VW will cause engine misfiring, hesitation, and stumbling when you hit the road. The misfire and the hesitation will depend on the extra ratio.

Lack of engine power

Once you have a compromised air-fuel ratio, it’ll affect the overall engine performance. Therefore, you’ll experience a loss of engine power depending on the severity of the P0172 code.

What Causes error code P0172?

Several parameters can cause the car computer to throw P0172 on ford or your specific car model. However, the most common factors are clogged or dirty Mass air flow sensors and lousy oxygen sensors. Other probable causes include;

  • Vacuum leak
  • Faulty fuel pressure regulator
  • The thermostat stuck in the open position
  • Malfunctioning throttle body position sensor
  • Defective manifold absolute pressure (MAP) sensor
  • Lousy fuel injectors leaking fuel inside the cylinders
  • Faulty or restricted fuel lines
  • Bad spark plugs
  • Clogged or dirty air filter
  • Lousy powertrain control module (not likely the case, though, possible)

Let’s look at the severity of the P0172 error code.

How serious is error code p0172?

The severity of the P0172 diagnostic trouble code is moderate. It’ll hardly stop you on the road. However, the vehicle will not pass state emission inspection because it will project the engine warning light. Secondly, if the fuel mixture is too rich, the engine will produce black smoke from the tailpipe. Of course, this is harmful to the environment.

Common P0172 diagnosis mistakes

The most common diagnostics mistake among technicians is not checking the temperature sensor and other cooling system components. For instance, the car engine needs more fuel when it is cold to warm the engine to the normal operating temperature.

Therefore, if the coolant temperature sensor sends a false signal to the PCM, the powertrain control module may assume the engine is cold and send more fuel to warm it at the average operating temp. That said, it is important to ensure the temp sensor works fine before replacing any components.

How to diagnose

If you are familiar with the underhood components in your car, you can save the repair cost and diagnose the fault at home. However, P0172 on Jeep and other vehicles is an intermediate repair task. So, if you’re a beginner, contact your mechanic to track the culprit and proffer solutions. 

The diagnosis requires a scan tool and other common mechanical tools. It is also a labor-intensive and time-consuming task for DIYers.

Items and tools needed

Step 1: Scan the vehicle with a scan tool

Locate the OBD port on the vehicle and plug in your scan tool. Next, scan the vehicle and ensure no other code is present. If the scanner pulls out other codes, fix them before continuing to the next step.

Step 2: Examine the vacuum hoses

Examine the vacuum hoses and ensure there’s no leak in all the vacuum hoses and lines. If there’s a leak on the vacuum lines, you’ll hear a hissing sound around that area. A beginner will find it difficult to hear the hissing sound while the engine is running. That said, you need to avoid any distractions and pay rapt attention.

Step 3: Inspect the exhaust system and MAF sensor

If there’s no vacuum leak, inspect the mass air flow (MAF) sensor. First, check if it is dirty and clean it with contact or MAF sensor cleaner. After that, check if the air filter is dirty or clogged.

A clogged air filter will restrict airflow and cause the ECM to throw P0172. So, first, clean or replace a clogged air filter. After that, inspect the exhaust components. If an exhaust leak is present in the system, you’ll notice a soot buildup on the leaking area.

Step 4: Test the fuel pressure

Examine the fuel systems, especially the injectors. A leaking injector will inject excessive fuel inside the combustion chamber. Examine the injectors one at a time and ensure they have the same pressure. If any of the injectors are spraying excess fuel, replace them.

Step 5: Inspect the O2 sensors and the spark plugs

If you’re still experiencing rich fuel on bank 1, replace the O2 sensor and erase the logged error code.

After fixing the P0172 on Chevy or your respective vehicle model, inspect the spark plugs. If you see soot on the plug tips, clean the plugs or replace them.

p0172 toyota

How do you fix P0172?

The first approach to fixing a P0172 error code is to track the culprit using a high-end scan tool. However, if you don’t have the confidence to track and fix engine running rich issues, contact your mechanic for a professional diagnosis and repair.

An experienced mechanic will save you time and help you avoid spending money on the wrong parts. In addition, most service technicians will offer you fair pricing and a 12months warranty to solidify their work.

Approx. Repair Cost

P0172 is a time-consuming and labor-intensive repair job. However, the cost of the replacement parts is fair enough. Here are the probable parts you will replace to fix the problem.

  • Clean mass air flow sensor $75 to $100
  • Replace mass air flow sensor for $150 to $200
  • Vacuum leaks $100 to $150
  • Oxygen sensor $100 to $200
  • Fuel pressure regulator $150 to $250


Q: Can a dirty air filter cause a P0172 code?

A clogged or dirty air filter is a common cause of a P0172 and the easiest component to check when your car computer detects too much fuel or too little air inside the engine. In fact, the air filter should be the first thing you should check and clean or replace it, depending on how dirty it is.

Q: Can a vacuum leak cause P0172?

A vacuum leak is another common cause of a P0172 code. Identifying vacuum leaks requires a professional mechanic to visually track where the leak comes from or use a smoke machine. Here’s what a vacuum leak does; it introduces unmeasured air into the air-fuel mixture.

Once the engine control unit detects it, it’ll try to compensate by sending additional fuel into the system. And once this adjustment becomes too much, it will trigger error code P0172. 

Q: Can a bad O2 sensor cause a P0172 code?

The O2 sensor and the air-fuel sensor determine how much fuel the ECM needs to inject into the engine. If the O2 sensor fails, it may send a false signal to the ECM and cause it to throw a P0172 error code. However, it can also tell the computer that the engine is running lean.

Q: Can an exhaust leak cause a P0172 code?

An exhaust leak will cause the car computer to throw a P0172 error code. Let’s get this straight. Exhaust leaks will not typically cause the powertrain control module to log the P0172 error code unless the engine tries to compensate and inject more than enough fuel into the engine.

Ordinarily, a bad exhaust leak will cause a P0171 fault code, which means the O2 sensor signals the car computer of excess oxygen in the engine.

Q: Can a bad spark plug cause P0172?

A bad spark plug will cause the ECM to log error code P0172 on Audi and other vehicles. If a spark plug fails and cannot burn fuel properly, the ECU will detect the unburnt fuel and think the engine is running rich. As a result, it’ll register P0172 on the memory and display a check engine light to notify the driver of imminent issues in the system.

Final Words

By now, you have seen the meaning, causes, symptoms, diagnosis mistakes, and how to diagnose and fix the problem flawlessly. You can track and rectify the issues yourself. In addition, because several issues can cause the car computer to log P0172, it can be challenging to track and fix the cause.

Therefore, it is ideal to start with the cheapest and easiest options. First, check if the air filter is clogged and examine the mass air flow sensor. After that, walk down to the costly and unlikely causes.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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