#OBD Codes
P0174 – Fuel trim. System too lean. Bank 2
Technical definition
Fuel trim, bank 2. System too lean.
What does P0174 mean?
The P0174 trouble code indicates that your vehicle's engine runs too lean on bank 2. This occurs when the air-fuel mixture contains too much air and not enough fuel.
In engine configurations, such as inline-6, flat-6, V6, V8, V10, W8, the exhaust system is divided into two banks. "Bank 2" refers to the bank on the side opposite the first cylinder.
Exhaust system types. Source: densoautoparts.com
This imbalance in the air-fuel mixture can occur for several reasons, including faulty oxygen sensors, problems with the mass air flow sensor (MAF), intake or fuel system issues, and vacuum leaks.
What is the air-to-fuel ratio?
Air-to-fuel ratio (AFR) is the proportion of air to fuel in an engine's combustion mixture. A gasoline engine's ideal (stoichiometric) ratio is 14.7:1. This means it takes 14.1 liters of air to burn one liter of gasoline completely.
The upstream oxygen sensor measures the AFR and provides feedback to the engine control unit (ECU).
The air-to-fuel ratio graph below illustrates how AFR varies under different engine conditions. At the stoichiometric ratio, the engine is running efficiently. Any deviation, either too lean or too rich, will affect performance and may trigger the check engine light along with diagnostic trouble codes such as P0171 or P0174.
Engine power and fuel consumption dependence of air/fuel ratio (lambda)
How serious is P0174?
The P0174 code can be critical if you don't act fast enough. Especially on forced induction engines. You might not notice it at first, but it can cause very costly damage over time.
If you don't fix it, a lean condition will result in improper combustion and engine knocking. In addition, a lean mixture burns very hot, which can even melt engine internals and damage the ceramic honeycomb inside the catalytic converter.
This extreme heat may also severely damage components such as pistons, piston rings, spark plugs, cylinder heads, and valves.
Can I still drive with P0174?
While you can still drive with a P0174 code, you should avoid doing so. If necessary, head to the nearest repair shop, but proceed slowly, avoiding rapid throttle application and high engine speeds.
Keep in mind that while driving, you may experience reduced engine power and possible stalling.
Symptoms of P0174
When your vehicle has a P0174 code, there are several symptoms you might notice. These include:
- Engine stalling or rough idling
- Reduced power and poor acceleration
- Engine self-preservation mode (limp mode)
- Difficulty starting the vehicle
- Check engine light is on
- Unusual engine noises, such as knocking or pinging
Common causes of P0174
Air intake system issues
A P0174 fault code typically occurs when the engine gets too much air. This is often a result of problems in the intake system, such as:
- Holes or cracks in the air intake system after MAF sensor
- Vacuum leak in the intake manifold, vacuum hoses, or positive crankcase ventilation (PCV) system
- Cracked or loose intake manifold gasket
These issues allow unmeasured air to enter the combustion chamber. When excess air enters through the gaps, the engine's sensors can't measure it accurately, and this disturbs the air-fuel ratio.
Fuel system issues
Fuel system issues can also cause a lean mixture by limiting the fuel supply to the engine. Common fuel-related causes include:
- Dirty fuel filter
- Clogged fuel injectors
- Low fuel pressure due to weak fuel pump
- Faulty fuel pressure regulator
Sensor related issues
Problems with sensors can also cause P0174 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
- Exhaust leaks upstream of the O2 sensor will distort sensor readings
How to troubleshoot P0174
With the right diagnostic tools, such as OBDeleven, and some knowledge, identifying the cause of a P0174 code can be straightforward.
To avoid wasting money repairing components that aren’t faulty, use these tips:
Scan fault codes
Use a diagnostic scanner to confirm the P0174 code and look for any additional codes that may provide further clues to the underlying problem. These include:
- P0170: Fuel trim malfunction indicates an issue with how the engine adjusts the air-fuel mixture
- P0171: Indicates the engine is running too lean on Bank 1. This often appears alongside P0174 and can point to a shared issue affecting both banks.
- 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
- 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
Use your scanner to clear fault codes. Then, drive your car for at least 15 minutes to fully warm up the engine – some issues only show up when the engine is hot.
Also, try driving at different engine speeds, as sometimes this code only shows up at high RPMs.
After the drive, perform another scan to see if the issue reappears. If it does, it means you’ll need to dig deeper and fix the problem.
Check freeze frame data
The freeze frame data captured when the fault code occurs can provide valuable insight. This may include mileage, date, engine load, temperature, rpm, and other parameters that help you understand the conditions that triggered the code.
For example, you can use the date or mileage data to recall the driving conditions – whether it was highway, city, track driving, or a day with extreme weather, like unusually hot or cold temperatures.
Analyze fuel trim live data readings
To better understand the severity and persistence of the issue, check the fuel trim live data values. Live data refers to real-time information provided by the vehicle's sensors.
In the OBDeleven VAG mobile app, you can find these values under the engine control unit details.
OBDeleven live data feature
Typically, fuel trim values range from -5% to +5%.
High positive values indicate the engine compensates for a lean condition by adding more fuel. In contrast, high negative values indicate that the engine reduces fuel to correct a rich condition.
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 P0174 code. Consider looking at:
- Mass airflow 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.
- Exhaust Gas Recirculation (EGR) valve position. Check the EGR valve to ensure it is functioning properly and not causing incorrect air-to-fuel ratios.
- Fuel pressure reading. Ensure the fuel pressure regulator is within the range specified by your vehicle's service manual.
Regularly change the fuel filter
A fresh fuel filter ensures a steady flow of clean fuel to the engine. It prevents contaminants from clogging the fuel injectors.
Experts generally recommend changing the fuel filter every 30,000 to 40,000 kilometers (20,000 to 25,000 miles), but always check your vehicle owner’s manual for specific intervals.
Clean MAF sensor
A dirty MAF sensor is a common cause of the P0174 fault code. Cleaning it is a simple process that can restore accurate air measurements.
Here's how to clean the MAF sensor:
- Locate the MAF sensor. It typically sits 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.
- 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 is very fragile. Let it dry completely.
- Reinstall the sensor. After it dries, reinstall the sensor and reconnect the electrical connector.
MAF sensor. Source: mastermechanicsauto.com
FAQ
Which oxygen sensor triggers P0174?
The upstream O2 sensor in bank 2 triggers the P0174 code when it detects a lean air-fuel ratio.
How much does it cost to fix code P0174?
The cost to fix a P0174 code varies widely. In some cases, simply reconnecting a vacuum hose, which costs nothing if you do it yourself, can solve the issue. However, more severe problems, such as faulty fuel injectors or a failed fuel pump, can cost several hundred to thousands of dollars.
Ignoring the issue for too long can lead to severe damage, including engine or catalytic converter failure, which comes with extremely high repair costs.
What causes P0171 and P0174 at the same time?
When P0171 and P0174 codes appear together, a problem affects both engine banks. Common causes include vacuum leaks, a faulty or dirty MAF sensor, or fuel delivery problems such as a weak pump or clogged filter. Unmeasured air from leaks in the intake system can also trigger both codes.
How does a faulty fuel pump cause a P0174 code?
A weak or failing fuel pump can cause insufficient fuel delivery to the engine, especially when driving fast or under a heavy load. This lack of fuel causes the engine control unit to detect a lean mixture and trigger the P0174 trouble code.
Why is it important to use only specified MAF sensor cleaners?
Other cleaners may contain grease and other substitutes that don't evaporate. This leaves a film on the sensor wire, which disrupts the measurements. Also, the wire is very sensitive to abrasives.
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.