Rick's Free Auto Repair Advice

P0171 trouble code

How to diagnose and fix a P0171 trouble code

What is a P0171 trouble code?

The official definition of a P0171 code is: System too Lean (Bank 1). That means the pre-catalytic converter oxygen sensor is sensing a constantly lean condition from the exhaust entering the catalytic converter. A lean condition is one where there’s too much air and not enough or no unburned fuel. It is one of the most common trouble codes.

First, you have to understand that the PCM/ECM is always playing catch-up, trying to correct it’s air/fuel calculations after the fact. The PCM/ECM’s air/fuel calculations are NEVER perfect because you’re always moving your foot on the accelerator pedal, the vehicle

short term fuel trim

Scan tool fuel trim readings

is always encountering hills, slopes and headwinds. So it calculates an air/fuel mixture and then uses the pre-cat oxygen sensor readings as a check on how well it did. In other words it expects to see a rich exhaust from overshooting the air/fuel mixture and a lean condition from undershooting the air/fuel mixture. It’s constantly trying to correct, but it never gets its spot on.

So, when the PCM/ECM receives a constantly lean condition report from the pre-cat oxygen sensor, it knows that something is really wrong.

What causes a P0171 trouble code?

The PCM/ECM measures all air coming into the engine and THEN calculates the proper amount of fuel based on the input from the accelerator pedal, the temperature of the incoming air, the temperature of the engine, atmospheric pressure or mass airflow readings (air density), engine speed and gear.

When an engine develops a vacuum leak or a tear in the air duct from the air filter box to the throttle body, the engine sucks in UNMETERED air—air that got into the engine that the computer doesn’t know about. That causes a lean condition in the exhaust, which is read by the pre-cat oxygen sensor.

The PCM/ECM responds by ADDING fuel and then checking the pre-cat oxygen sensor readings again. The process of adding fuel is called short term fuel trim. The PCM/ECM can add or subtract around 25% more or less fuel to bring the exhaust back into the proper range. If the PCM/ECM hits its maximum adjustment, it start adding more fuel, using its long term fuel trim strategy. When it reaches that maximum, its game over; time to set the checking engine light and store a P0171 trouble code.

Finally, you may not have an air leak at all on the air intake side; you may have an exhaust leak that’s allowing outside air into the exhaust right before the oxygen sensor. That outside air would skew the sensor reading to constantly lean.

What are the symptoms of a P0171 code?

• Poor gas mileage

• Rough idle

• Less power and hesitation on acceleration

• Harsh shifting on some brands with automatic transmission.

Most likely causes of a P0171 code

• Broken, split, unhooked or missing vacuum line

• Tear or split in the air duct running from the

fix engine misfire

Tear in air intake hose that allows unmetered air into the engine

air filter box to the throttle body

• Upper intake plenum Gasket leak

• Lower intake manifold gasket leak

How to locate the air leak and fix a P0171 code

• Check all vacuum lines for integrity and proper hook up. A P0171 code can appear right after an oil change because the tech checked your air filter and didn’t properly reconnect the vacuum lines or the air duct.

• If all vacuum lines check out, spray carb cleaner around upper and lower intake manifold gaskets and listen for increased RPMs or monitor pre-cat oxygen sensor readings with the live data feature on your scan tool. You should see a rapid rise and fall in the readings.

• Check for exhaust leaks ahead of the pre-cat oxygen sensor.

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat


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