Trouble code P0171: System Too Lean can be one of the most difficult trouble codes to troubleshoot and resolve.
In this tutorial I'll shed some light on what this trouble code means and some of the tests you need to perform to find its cause (and solution).
Contents of this tutorial:
You can find this tutorial in Spanish here: Código P0171 ¿Qué Significa? (1995-1997 2.2L Honda Accord And Odyssey) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 2.2L Honda Accord (DX, EX, LX): 1995, 1996, 1997.
- 2.2L Honda Odyssey: 1995, 1996, 1997.
What Does Trouble Code P0171 Mean?
In a nutshell, a trouble code P0171 means that the engine is not getting enough fuel to run optimally.
To be a bit more specific: A P0171 codes tells you that the air/fuel mixture the engine is running on is too lean.
What's a 'lean air/fuel mixture, I'll explain. To run optimally, the engine needs 14.7 grams of air for every 1 gram of fuel.
This 14.7:1 ratio of air and fuel is known as the stoichiometric air/fuel mixture. At this ratio of air and fuel, the engine will produce an optimal amount of power and produce less pollutants.
When the air/fuel mixture is considered 'lean', one of two things is happening:
- The amount of air entering the combustion chambers is greater than the 17.4 grams of air needed to maintain the 14.7:1 ratio.
- The amount of fuel entering the engine is less than the 1 gram of fuel needed to maintain the 14.7:1 ratio.
On the opposite end, when the amount of fuel is greater than the 1 gram of fuel needed to maintain the stoichiometric air/fuel mixture, the air/fuel mixture is considered 'rich'.
Common Symptoms Of A P0171 Trouble Code
A lean air/fuel mixture will have negative consequences on engine performance. You're gonna' see one or more of the following symptoms:
- The check engine light is on.
- Lack of power when accelerating the engine under load.
- Bad gas mileage.
- Rough idle.
- Explosions coming from the intake manifold when accelerating the engine under load.
- Engine knock (pinging) when accelerating the engine under load.
What Causes A P0171 Trouble Code?
Any component that, when it fails, can cause the air/fuel mixture to turn lean will cause a P0171 trouble code. Here are the most common:
- A bad fuel pump that is producing low fuel pressure.
- A dirty/clogged fuel filter.
- Dirty/clogged fuel injectors.
- A bad fuel injector.
- Vacuum leaks.
- A bad oxygen sensor (stuck reporting a rich air/fuel mixture).
How To Diagnose And Repair A P0171 Trouble Code
Repairing a trouble code P0171 involves finding and replacing the bad component that is causing the lean air/fuel mixture.
The bad component is either causing not enough fuel to enter the engine or causing too much air to enter the engine.
To find the bad component, I recommend the following tests:
- Check the fuel pump's pressure with a fuel pressure gauge. The following tutorial will help you with this test:
- Check and replace the fuel filter.
- Check for bad or clogged fuel injectors.
- Check for vacuum leaks.
- Check all vacuum hoses for damage and proper connection.
- Check for vacuum leaks around the intake manifold gasket.
- The following tutorial is a very good primer on how to locate a vacuum leak:
- Check the performance of the upstream and downstream oxygen sensors.
More 2.2L Honda Accord Tutorials
You can find a complete list of 2.2L Honda tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- How To Test The MAP Sensor (1994-1997 2.2L Accord, Odyssey, Prelude).
- How To Test The Head Gasket (1992-1997 2.2L Accord, Odyssey, Prelude).
- How To Test The Engine Compression (1992-1997 2.2L Accord, Odyssey, Prelude).
- How Often Should I Replace The Spark Plugs? (2.2L Honda Accord).
If this info really saved the day, buy me a beer!