Error Code P0171

So your ‘Check Engine’ light is on and it reads error code P0171. What does it mean? Most people mistake it as oxygen sensors being bad. But the truth of the matter is, it does not identify as the oxygen sensor as bad. But what is it exactly? This entry will break it down for you.

Error code P0171 means there is incorrect ratio of air and fuel on the car’s engine system.


The computer (Powertrain Control Module) of the vehicle constantly monitors the fuel delivery system’s adaptive fuel strategy to ensure good condition on the engine at an optimum air-to-fuel ratio of 14.7:1. The computer automatically adjusts its injector pulse to help regulate the amount of fuel that gets sent to the engine. The oxygen sensor then relays the information to the computer and uses it to determine how much fuel is needed. The computer will then set the fuel flow (and even other related operating characteristics of the system) to maintain the right ratio of air and fuel combination. This loop goes on and on as the engine runs. The error code P0171 appears when the computer has reached rich calibration limit and can’t maintain the right ratio of air and fuel.

Common Symptoms

  • Check Engine Light is On
  • Lack of Power
  • Spark Knock
  • Roughness When Vehicle is Idle
  • Surge Upon Acceleration
  • Surge Excessive Fuel Consumption

Possible Causes

  • Problem on the MAF (Mass Air Flow) sensor
  • Downstream vacuum leaks of the Mass Air Flow sensor
  • Possible cracks or damage on the vacuum connection or PCV line
  • Stuck open or faulty PCV line
  • Faulty or failed oxygen sensor
  • Failure in the fuel injector
  • A dirty filter resulting in low fuel pressure
  • Exhaust leak of the first oxygen sensor and engine

How to Check

When you see the ‘Check Engine Light’ is on, use you OBD II scanner to determine the error code. If the reader shows P0171 then that means a lean engine error code has materialized in the system. This lean code is enabled when the computer detects maxed out of enrichment adjustments.

How to Fix

Things You’ll Need

  • Code Reader
  • Flashlight
  • Protective gloves and eyewear
  • Socket Set
  • Wrench

First and foremost, start by parking your vehicle on a flat ground and use an emergency brake set.

Test for Vacuum Leaks

Unmetered air that enters the engine’s intake system can lead to lead mixture code. Thus, make sure you check for signs of cracks or dilapidation throughout the vacuum lines. Of course, make sure you check the integrity of each end of the vacuum hoses.

The intake system uses gaskets to seal its components. Damaged gaskets can result in a vacuum leak, which will show code failure. A carburetor cleaner can help see any potential leaks by simply spraying it around the surface of the gasket. If there’s a leak, a rise in engine’s idle speed will be observed.

Test for Exhaust Leak

The exhaust system manages both pressure and vacuum cycles the engine runs. A leak in the exhaust will let fresh air get into the system through exhaust vent’s negative pressure side, causing the oxygen sensor to show a lean reading. To check for vacuum leaks, ask someone to start the engine and hold the idle for you at 1500 rpm. Then, listen for any noises in the exhaust that comes from the system, including head pipes and manifolds. Also, black soot can indicate exhaust leak, so make sure you look for them at any point as well.

Test the Service Mass Airflow Sensor MAF

After driving for miles, the hot wire (filament of mass air flow sensor) can be contaminated due to air impurities, which can lead to false readings. Remove these deposits and clean the filament by using a sensor-safe carburetor cleaner and allow the sensor to get the right reading.

Test the Oxygen Sensor

Verify the functionality of the oxygen sensor’s exhaust system. If the sensor fails, it will give incorrect feedback information, which often shows lean mixture code.

Test the Sparkplug

When the engine misfires, almost-clean oxygen charge is sent to the exhaust system. This will cause the system to get ‘lean’ reading from the O2 sensor. This however, can be easily fixed through tune-up.

Test the Air Intake Boot Replacement

The air intake boot transports engine air from the MAF sensor up to the throttle accelerator. If it fails, it will show false reading to the mass air flow sensor tool code.

After making these repairs, clear the trouble codes and let the computer monitors reset by driving the vehicle at around 30 mph to get the right functionality.

For more information about different OBD II error codes, browse through our website. If you need professional assistance, feel free to talk to Contact Us.

Shops Near You

Subscribe to our mailing list