P0172 Trouble Code 1995, 1996, 1997 2.2L Honda Accord DX, EX, LX

Trouble code P0172: System Too Rich lets you know that too much fuel is being injected into the engine, but doesn't really tell you what component has failed.

In this tutorial I'll shed some light on what this trouble code means and some of the tests you can perform to find its cause (and solution).

En Español You can find this tutorial in Spanish here: Código P0172 ¿Qué Significa? (1995-1997 2.2L Honda Accord And Odyssey) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

  1. 2.2L Honda Accord (DX, EX, LX): 1995, 1996, 1997.
  2. 2.2L Honda Odyssey: 1995, 1996, 1997.

What Does Trouble Code P0172 Mean?

In plain English, a trouble code P0172 means that the engine is getting too much fuel to run optimally.

To be a bit more specific: A P0172 codes tells you that the engine is being injected with too much fuel and that the fuel injection computer can no longer compensate for this extra fuel.

In technical terms, this means that the engine's air/fuel mixture is too rich in fuel.

What's a 'rich 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 'rich', one of two things is happening:

  1. The amount of air entering the combustion chambers is less than the 17.4 grams of air needed to maintain the 14.7:1 ratio.
  2. The amount of fuel entering the engine is more than the 1 gram of fuel needed to maintain the 14.7:1 ratio.

On the opposite end, when the amount of air is greater than the 14.7 grams needed to maintain the stoichiometric air/fuel mixture, the air/fuel mixture is considered 'lean'.

Common Symptoms Of A P0172 Trouble Code

A rich air/fuel mixture will have negative consequences on engine performance. You're gonna' see one or more of the following symptoms:

  1. The check engine light is on.
  2. Black smoke coming out of the tailpipe -especially when accelerating the engine under load.
  3. Lack of power when accelerating the engine under load.
  4. Bad gas mileage.
  5. Rough idle.
  6. Rotten egg smell coming out of the tailpipe.

What Causes A P0172 Trouble Code?

Any component that, when it fails, can cause the air/fuel mixture to turn excessively rich will cause a P0172 trouble code. Here are the most common:

  1. A bad fuel pressure regulator (leaking fuel into its vacuum hose).
  2. Severely worn or carbon-blocked spark plugs.
  3. A dirty/clogged air filter.
  4. Leaking fuel injectors.
  5. A bad oxygen sensor (stuck reporting a lean air/fuel mixture).

How To Diagnose And Repair A P0172 Trouble Code

Repairing a trouble code P0172 involves finding and replacing the bad component that is causing the rich air/fuel mixture.

The key is to remember that the failed (bad) component is either causing not enough air to enter the engine or causing too much fuel to enter the engine.

To find the bad component, I recommend the following tests:

  1. Check for a dirty air filter.
  2. Check the fuel pressure regulator. Specifically, unplug its vacuum hose and check if gasoline is coming out.
  3. Check all spark plugs for severe wear and tear or carbon blockage.
  4. Check for misfire trouble codes. If present, troubleshoot and resolve them.
  5. Check the performance of the upstream and downstream oxygen sensors.
  6. Check for bad or clogged fuel injectors.
  7. Check engine compression.

More 2.2L Honda Accord Tutorials

You can find a complete list of 2.2L Honda tutorials in this index:

  1. Honda 2.2L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

  1. How To Test The MAP Sensor (1994-1997 2.2L Accord, Odyssey, Prelude).
  2. How To Test The Head Gasket (1992-1997 2.2L Accord, Odyssey, Prelude).
  3. How To Test The Engine Compression (1992-1997 2.2L Accord, Odyssey, Prelude).
  4. How Often Should I Replace The Spark Plugs? (2.2L Honda Accord).
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