Repair Topics > P Codes and their fixes > Understanding the P0171 Code: Causes and Solutions

Understanding the P0171 Code: Causes and Solutions

Troubleshooting the P0171 Code: A Comprehensive Guide

A P0171 code is common and one that causes a lot of frustration for drivers and DIYers alike. This code indicates a problem related to the fuel system’s air-to-fuel ratio. It’s defined as: P0171 – Fuel Trim System Too Lean (bank 1). That means the oxygen sensor on bank 1 is sensing a lean exhaust mixture.

What is the P0171 Code in plain-speak?

In simple terms, it means that the engine is getting either too much air and/or too little fuel compared to the ideal air-to-fuel ratio of around 14.7:1 (air-to-fuel ratio). This imbalance typically results in an engine running lean, which can lead to performance issues, increased emissions, and potential damage if left unresolved.

Symptoms of a P0171 code

• Check Engine Light
• Low engine power
• Poor idle
• Stalling
• Engine misfire
• Spark/Engine Knock
• Possible hard start or no start

What causes a P0171 code?

• Vacuum leak— This is the most common cause and is due to a tear in the air intake duct from the air filter box to the throttle body, a cracked or detached vacuum line, or a more serious intake manifold leak.
• Faulty Oxygen (O2) Sensor— The oxygen sensors monitor the amount of oxygen in the exhaust gases and provide feedback to the ECM/PCM to adjust the air-fuel mixture. A faulty O2 sensor can cause incorrect adjustments, resulting in a lean condition.
• Problems with the mass airflow sensor (MAF)— The MAF sensor is the primary sensor used by the PCM to determine the proper air-to-fuel mixture. If it’s not sensing properly, you can get a P0171 code.
• Clogged or defective fuel injectors— Clogged or faulty injectors deliver too little fuel to the cylinder
• Exhaust leak— Any leak that occurs ahead of the upstream oxygen sensor will let too much outside air into the exhaust, causing this code.
• Weak fuel pump— If the fuel pump isn’t delivering enough fuel or fuel at the right pressure, the air: fuel mixture will be too lean.
• Broken fuel pressure regulator— A fuel pump pressure regulator can cause a low fuel pressure issue
• Blocked fuel filter— A clogged fuel filter can prevent the engine from getting enough fuel.
• Faulty powertrain control module— This is pretty rare and generally sets PCM codes as well.
• Clogged catalytic converter— A clogged converter or muffler prevents proper airflow into and out of the engine.

Step-by-step guide to diagnose a P0171 code

1) Start by checking for vacuum leaks or unmetered air entering the engine. Examine the plastic air duct that runs from the air filter box to the throttle body. Check for cracks or tears. If you find any, replace it.
2) Next, check for disconnected or cracked vacuum lines.
3) Spray carburetor cleaner around the intake manifold gasket to see if any gets sucked in. That would indicate a bad intake manifold gasket.
4) Check for exhaust leaks. See this post for the procedure to check for an exhaust leak.
5) Clean the MAF sensor. See this post for instructions on how to clean a MAF sensor.
6) Check for proper oxygen sensor performance.
7) Perform a fuel pressure and fuel volume test.
8) Check for fuel pressure drop while activating each fuel injector.
9) Check the engine coolant temperature sensor and the ambient air temperature sensor. The PCM uses data from these sensors to calculate fuel. If it’s off, the PCM can’t determine the proper fuel delivery.