#OBD Codes
P0172 – Fuel trim. System too rich. Bank 1
Technical definition
Fuel trim, bank 1. System too rich.
What does P0172 mean?
The P0172 code pops up when your engine runs too rich on Bank 1 – meaning there’s too much fuel or not enough air in the air-fuel mixture.
This imbalance can happen for various reasons, such as a faulty oxygen sensor, issues with mass air flow (MAF) sensors, fuel system problems, or vacuum leak. P0172 often shows up alongside P0175, which means the same issue is happening on Bank 2.
What does "Bank 1" mean?
In engine configurations, such as V-shaped, W-shaped, or flat, the cylinders are grouped into banks. "Bank 1" refers to the side of the engine containing the first cylinder. While the location of the first cylinder depends on the engine’s design and manufacturer, it’s typically closest to the front of the engine near the drive belt.
V-shaped engine banks. Source: densoautoparts.com
If your engine has all its cylinders in a single line, everything belongs to bank 1.
What is the fuel ratio?
The air-to-fuel ratio (AFR) measures how much air mixes with fuel in your engine. For gasoline engines, the perfect (stoichiometric) ratio is 14.7:1. This means it takes 14.1 kg of air to burn 1 kg of gasoline completely.
The upstream oxygen sensor measures the air-fuel ratio (AFR) and provides feedback to the engine control unit (ECU) to keep the mixture balanced.
The air-fuel ratio graph below illustrates how the AFR varies under different engine conditions. The stoichiometric ratio enables the engine to run most efficiently, ensuring no fuel or oxygen gets wasted. Any significant deviation, whether too lean or too rich, will affect engine performance and may trigger the check engine light along with diagnostic trouble codes P0171-P0175.
Engine power and fuel consumption dependence of air/fuel ratio (lambda)
How serious is P0172?
While P0172 might not shut your engine down immediately, ignoring it isn’t a good idea. Over time, running too rich can lead to carbon buildup on your engine’s pistons, valves, spark plugs, and even in the exhaust system. This buildup reduces efficiency and reliability, not to mention it’s tough on your wallet with poor fuel economy.
Additionally, the excess unburned fuel can ignite in the exhaust system and melt the ceramic core of the catalytic converter, one of the most expensive components in your car.
Fixing the P0172 code promptly helps you avoid these headaches and helps keep your car running smoothly.
Can I still drive with P0172?
Yes, you can still drive with a P0172 code. But while the issue might not be obvious to you right away, those around you are likely to notice.
A rich air-fuel mixture means your engine injects more fuel than it can burn. This leads to a strong fuel odor coming from your exhaust, and if you drive a diesel car, expect clouds of black smoke trailing you – especially in older vehicles.
Moreover, excess fuel produces carbon monoxide (CO), a very toxic gas that harms the environment and your car's chances of passing an emissions test.
Common symptoms of P0172
When your vehicle has a P0172 code, there are several symptoms you might notice. These include:
- Check engine light
- Rough idle
- A strong fuel odor from the tailpipe
- Black smoke from the exhaust
- Poor fuel economy
- Misfires
- Reduced power and poor acceleration
- Engine self-preservation mode (limp mode)
- Carbon buildup on spark plugs
Common causes of P0172
Air intake system issues
A P0172 fault code typically occurs when the engine gets too little air. This is often a result of problems in the intake system, such as:
- Dirty air intake filter
- Vacuum leaks
Both issues restrict air intake to the combustion chamber.
Fuel system issues
Fuel system issues can also cause a rich mixture by oversupplying the engine with fuel. Common fuel-related causes include:
- Leaking fuel injector
- High fuel pressure due to pinched or clogged fuel return line
- Faulty fuel pressure regulator
Sensor related issues
Problems with sensors can also cause the P0172 code. For example:
- A faulty MAF sensor may send the wrong airflow data to the ECU
- Bad O2 sensor can misread exhaust gases and cause incorrect fuel adjustments
Other issues
- Clogging due to buildup or physical damage in these exhaust components: catalytic converter, exhaust pipe, and muffler
- Exhaust leak before the O2 sensor (more commonly fail by reading lean, not rich)
- Contaminated engine oil (too long since the last oil change)
- Circuit issues, such as loose connections and damaged wiring
- Issues with the engine control unit
How to troubleshoot P0172
To avoid wasting money fixing components that aren't broken, use these tips:
Scan fault codes
Using a diagnostic scanner, confirm the P0172 code and look for additional codes that may provide further insights into the problem. Other common trouble codes to watch for include:
P0101-P0104: Mass airflow sensor-related codes indicating issues with the sensor that could affect the air-fuel mixture
P0130-P0135: Oxygen sensor-related codes might signal problems with the sensors responsible for monitoring the air-fuel ratio
P0170: Fuel trim malfunction indicates an issue with how the engine adjusts the air-fuel mixture
P0175: Indicates the engine is running too rich on Bank 2. This often appears alongside P0172 and can point to a shared issue affecting both banks.
P0300-P0312: Misfire codes can suggest fuel delivery, ignition, or air intake issues that might contribute to a lean condition
Clear the codes and test drive
Firstly, use your scan tool to clear the P0014 fault code from your car.
- If the fault code immediately reappears: This indicates the issue is ongoing. Skip the test drive and continue troubleshooting other potential causes.
- If the codes clear successfully: Drive your car for about 15 minutes to fully warm up the engine and observe its behavior.
After the test drive, perform another scan to see if the code reappears. If it does, this confirms the problem persists and requires further investigation.
Analyze fuel trim live data readings
Live data tools, such as OBDeleven, enable you to monitor real-time intake fuel trim readings for Volkswagen Group vehicles. To access these values in the OBDeleven VAG app, connect to the vehicle and navigate to the control unit list. Under the "Live data" tab in the engine control section, you will find the fuel trim readings:
OBDeleven live data feature
Typically, fuel trim values range from -5% to +5%.
High negative values indicate the engine corrects a rich air-fuel mixture by reducing fuel injection. In contrast, high positive values indicate the engine adds more fuel to compensate for lean conditions.
Keep in mind it's crucial to look at live data parameters only when the engine is warm, as a cold engine will naturally run rich.
Further reading: How to read OBD live data
Check other live data parameters
Other related live data parameters can also help identify potential causes of the P0172 code. Consider looking at:
- Mass air flow sensor readings. Ensure the MAF sensor is providing accurate airflow measurements.
- Oxygen sensor voltage. Check the upstream O2 sensor for proper operation, typically fluctuating between 0.1 and 0.9 volts.
- Fuel pressure reading. Ensure the fuel pressure regulator is within the range specified by your vehicle's service manual.
- Injection amount deviation. For diesel engines, check fuel injectors deviation. This indicates how much the ECU adjusts the fuel injection quantity for each cylinder to ensure smoother operation. The deviation for each cylinder should not exceed ±3 mg/stroke.
Change air filter regularly
A clean air filter ensures steady airflow.
It is recommended to change the air intake filter every 24,000 kilometers (15,000 miles). However, if you often drive in a dusty environment, consider changing it more often.
Clean MAF sensor
MAF sensors are very sensitive to dirt and oil splashes in the air, which can be a cause of the P0172 fault code. Simply cleaning the sensor can restore its accurate air readings.
Here's how to clean the MAF sensor:
- Locate the MAF sensor. It's typically located between the air filter box and the throttle body.
- Disconnect the MAF sensor. Carefully unplug the electrical connector to avoid damage.
- Remove it. Use a screwdriver to remove the screws securing the sensor in place.
- Inspect the sensor. Look for physical damage, corrosion, or disconnections in the MAF sensor and its wiring.
- Clean the sensor. Spray the sensor wire only with a specialized MAF sensor cleaner. Oil-based solutions can damage the sensor. Don't touch the wire, as it's very fragile. Let it dry completely.
- Reinstall the sensor. After it dries, secure it in place and reconnect the electrical connector.
MAF sensor. Source: mastermechanicsauto.com
Check fuel lines
Inspect fuel lines for damage or pinches, especially the fuel return line, as restrictions can cause excessively high fuel pressure in the fuel rail.
Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.
Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.
Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.