How Do I Fix Code P0171 and P0174: 6 Quick Steps

Seeing a P0171 or P0174 code on your car can be stressful. I faced this problem with my vehicle, but I fixed it quickly by checking for vacuum leaks and cleaning the mass airflow sensor. In this article, I will show you exactly how to fix P0171 and P0174 with clear, practical methods that work. You can solve this issue without expensive repairs and have your car running smoothly again.

Check for Vacuum Leaks

One of the most common causes of P0171 and P0174 codes is a vacuum leak. Start by inspecting all vacuum hoses under the hood. Look for cracks, splits, or loose connections. Even a small leak can cause your engine to run lean, triggering these codes.

Spray a small amount of carb cleaner or soapy water around the hoses while the engine is running. If the engine revs change, you found a leak. Listen carefully to hissing sounds near connections. These are signs that air is entering where it shouldn’t.

Next, check the intake manifold gasket. Over time, gaskets wear out and let air in. You may notice rough idling or decreased fuel efficiency if this is the cause. Replacing the gasket is usually simple and can prevent these codes from returning.

If you find a vacuum leak, replace the damaged hose or seal. Make sure all connections are tight and secure. After repairs, clear the codes using an OBD2 scanner and test drive your car. The P0171 or P0174 code should disappear if the leak was the problem.

Inspect all vacuum hoses.
Spray carb cleaner or soapy water to detect leaks.
Check intake manifold gasket.
Replace damaged parts and clear codes.

Clean the Mass Airflow Sensor

A dirty or faulty mass airflow (MAF) sensor can trigger P0171 and P0174 codes. The MAF sensor measures the amount of air entering the engine. If it reads incorrectly, the engine will run lean and set a code.

Locate the MAF sensor, usually between the air filter and throttle body. Carefully remove it from the housing. Use a dedicated MAF sensor cleaner spray. Do not touch the sensor element with your fingers or other tools.

Spray gently to remove dirt, dust, and oil buildup. Let it dry completely before reinstalling. Reattach the sensor to the intake. Make sure the wiring harness is properly connected and secure.

After cleaning, clear the codes with an OBD2 scanner. Start the engine and check for smooth idling. Driving for a few miles can help the car relearn normal airflow patterns. This simple cleaning often fixes lean codes without replacing parts.

Locate and remove MAF sensor.
Use dedicated cleaner spray.
Let it dry completely.
Reinstall and clear codes.

How Do I Fix Code P0171 and P0174 Fast and Simple

Also Read: Can Fuel Pump Cause Check Engine Light? The Truth Find Out!

Inspect Fuel System and Pressure

Low fuel pressure can also cause P0171 and P0174 codes. First, check your fuel filter. A clogged filter restricts fuel flow, causing a lean condition. Replace it if necessary. Fuel filters usually last 30,000 to 50,000 miles.

Next, check the fuel pump. Listen for unusual sounds when turning the key on. You may also use a fuel pressure gauge to ensure it meets manufacturer specs. Low pressure can trigger lean codes and poor performance.

Inspect fuel injectors for clogging or leaks. Dirty or leaking injectors reduce fuel delivery. Cleaning or replacing them restores proper fuel mixture. A consistent fuel supply is key to resolving these codes.

Finally, reset the engine control unit using a scanner. Drive your vehicle and monitor the performance. If fuel delivery was the problem, the P0171 and P0174 codes should disappear, and your car will run smoothly again.

Check and replace fuel filter if needed.
Test fuel pump pressure.
Inspect and clean fuel injectors.
Reset ECU and test drive.

Examine Oxygen Sensors

Oxygen sensors play a crucial role in controlling air-fuel mixture. A failing sensor can send incorrect data to the engine control unit, causing P0171 and P0174 codes. Start by checking the front sensors near the exhaust manifold.

Use an OBD2 scanner to view live data from the sensors. Watch the voltage readings; they should fluctuate between 0.1 and 0.9 volts while the engine runs. A constant reading indicates a bad sensor.

Inspect wiring and connectors for damage or corrosion. Faulty wiring can mimic a sensor problem and trigger lean codes. Replace the sensor if needed and make sure connections are clean and tight.

After replacement, clear the codes and allow the car to adapt. Drive under different conditions to ensure the readings are stable. Proper oxygen sensor function helps the engine run efficiently and keeps these codes from returning.

Check front oxygen sensors.
Monitor live data with a scanner.
Inspect wiring and connectors.
Replace faulty sensors and clear codes.

Examine the Exhaust System for Leaks

Exhaust leaks can trick the engine into thinking it is running lean. Check around the exhaust manifold, catalytic converter, and pipes for leaks. Listen for hissing or unusual noises while the engine runs.

Spray soapy water on suspected areas while the engine is running. Bubbles indicate escaping exhaust gases. Small leaks near sensors can greatly affect readings and trigger P0171 or P0174 codes.

Inspect gaskets and clamps for wear or damage. Replace any faulty parts to seal the exhaust system. A tight exhaust prevents false readings and ensures proper fuel mixture.

After repairs, clear the codes and test drive your vehicle. Properly sealed exhaust prevents lean code errors. This is a simple method often overlooked but highly effective in fixing P0171 and P0174 codes.

Check exhaust manifold and pipes.
Spray soapy water to detect leaks.
Replace faulty gaskets or clamps.
Clear codes and test drive.

Check Engine Coolant Temperature and PCV Valve

Engine temperature affects fuel mixture. A faulty coolant temperature sensor can send incorrect readings, causing lean codes. Inspect the sensor for damage and test it with a multimeter. Replace if readings are out of spec.

The PCV (positive crankcase ventilation) valve can also cause lean conditions. A stuck or clogged PCV valve allows excess air into the intake. Remove and clean or replace it as needed. Proper function ensures stable fuel mixture.

After replacing the sensor or PCV valve, clear the codes with an OBD2 scanner. Drive the car and monitor engine performance. Lean codes should be gone, and idling will be smooth. Proper maintenance prevents these errors from returning.

Test coolant temperature sensor.
Replace if faulty.
Clean or replace PCV valve.
Clear codes and monitor engine.

Final Thoughts

Fixing P0171 and P0174 codes is easier than it seems. By checking for vacuum leaks, cleaning the MAF sensor, inspecting fuel and oxygen systems, sealing the exhaust, and checking sensors and PCV valve, you can restore proper engine performance. Follow these methods carefully, and your car will run smoothly again. Regular maintenance helps prevent these lean codes in the future.

Method	How to Do It	Time Needed	Notes
Vacuum Leaks	Inspect hoses and gaskets	20–30 min	Look for cracks and hissing
MAF Sensor	Remove and clean sensor	15 min	Let it dry completely
Fuel System	Check filter, pump, injectors	30–40 min	Replace or clean as needed
Oxygen Sensors	Test and inspect wiring	20 min	Replace faulty sensors
Exhaust System	Inspect for leaks	25–35 min	Replace gaskets or clamps
Coolant & PCV	Test sensor and valve	15–20 min	Replace if readings are off

Frequently Asked Questions (FAQs)

Is it safe to drive with P0171 or P0174 codes?

It is not recommended. Driving with these codes can cause poor fuel economy and engine performance. Long-term use may damage the catalytic converter. Addressing the cause quickly ensures safety and prevents bigger repair costs.

Can a dirty air filter cause P0171 or P0174?

Yes, a clogged air filter restricts airflow, which can trigger lean codes. Replacing or cleaning the filter restores proper airflow. Always check the air filter when diagnosing lean conditions. Proper airflow keeps the engine running efficiently.

Do I need an OBD2 scanner to fix these codes?

While not strictly necessary, an OBD2 scanner makes diagnosing easier. It allows you to read live data and confirm the exact cause. Clearing codes and monitoring sensor performance is faster and more accurate with a scanner.

Is it possible to fix these codes without replacing parts?

Sometimes yes, if the issue is dirt or minor leaks. Cleaning the MAF sensor or fixing a vacuum leak can solve the problem. Replacement is needed when parts are worn or damaged. Always inspect before buying new components.

Can low fuel pressure cause lean codes?

Yes, low fuel pressure reduces fuel supply to the engine. This causes a lean condition, triggering P0171 or P0174 codes. Checking the fuel filter, pump, and injectors ensures proper fuel delivery.

Do I need to check oxygen sensors first?

It is recommended to check for simpler causes first, like vacuum leaks and MAF sensor issues. Oxygen sensors can fail, but they are usually last to check. Testing sensors ensures accurate diagnosis before replacement.

Is the PCV valve important for lean codes?

Yes, a clogged or stuck PCV valve lets extra air enter the intake. This creates a lean condition, triggering P0171 and P0174 codes. Cleaning or replacing the valve helps restore proper air-fuel balance.

Can exhaust leaks trigger lean codes?

Absolutely. Exhaust leaks, especially near sensors, can cause false readings. This makes the engine think it is running lean. Repairing leaks prevents P0171 and P0174 codes from returning.