How To Test Misfire Trouble Codes (2003-2006 2.4L Honda Accord)

How To Test A Misfire Condition And Misfire Codes (2003, 2004, 2005, 2006 2.4L Accord And Element)

Testing a misfire code, or misfire codes, or a rough idle condition on your 2.4L Honda Accord or Element can seem difficult, especially since so many things can cause the issue. Not only that, but the misfire code lighting up your check engine light (CEL) doesn't tell you exactly what's bad.

In this tutorial, I'm going to explain in some detail the most common causes of misfires and misfire codes (P0300, P0301, P0302, P0303, P0304) and more importantly, I'm also gonna' offer you a simple diagnostic strategy that I'm certain will help you ‘nail down’ the cause of the misfire condition, misfire code, or rough idle condition your Honda is experiencing.

In Spanish You can find this tutorial in Spanish here: Cómo Probar Una Falla En Cilindro (2003-2006 2.4L Honda Accord) (at:

What Is A Misfire Condition?

A misfire condition, in non technical terms simply describes an engine that is not running on all cylinders. So, if your 2.4L 4 cylinder Honda Accord or Element is having just one cylinder misfiring you're gonna' feel it. Your Honda Accord will have one or more of the following symptoms:

  1. The check engine light (CEL) will be on.
  2. One or more misfire codes will be stored in your Honda's PCM memory.
    • P0300 Random Cylinder Misfire.
    • P0301 Cylinder #1 Misfire.
    • P0302 Cylinder #2 Misfire.
    • P0303 Cylinder #3 Misfire.
    • P0304 Cylinder #4 Misfire.
  3. Sometimes, even tho' the engine is suffering a bonafide misfire, no misfire codes are registered and no Check Engine Light (CEL) comes on.
  4. Lack of power upon acceleration.
  5. Smell of unburned gas exiting the tail pipe.
  6. Rough idle and may stall.
  7. Cranks but does not start.
  8. Will not pass the emissions tests.
  9. Bad gas mileage.

Although the misfire codes don't tell you what exactly is the cause of the misfire or rough idle condition, there is a way to find out exactly what is causing it.

One of the most important things you need to know, to successfully diagnose a misfire or rough idle condition, is what causes a misfire. Let's go to the next subheading and find out.

What Causes A Misfire Condition?

To successfully diagnose a misfire code or condition, we need to know that each cylinder needs 3 very specific things to run/operate at their peak. If one of these 3 things is missing then the engine will experience a misfire or rough idle condition. These 3 things are:

  1. Air
  2. Fuel
  3. Spark

As mentioned before, if any of the above are missing in a specific engine cylinder then that cylinder will go ‘dead’, so to speak, and misfire. With this knowledge under our belts, let's take a look at how each one is delivered to the engine:

Ignition System: The ignition system is responsible for the production and delivery of spark. The ignition system is usually the culprit behind a Misfire.

The usual suspects (that cause a misfire) are:

  1. Bad spark plugs.
  2. Carbon tracks on the spark plug and spark plug boot.
  3. Bad Coil-On-Plug (COP) ignition coils.
  4. Oil dripping (from the valve cover) onto the spark plugs and COP ignition coil boots.

The good news is that all of the components that make up the ignition system can be tested, and at the end of the article I'll show you where to find the How To test articles.

Fuel System: The fuel system is responsible for the delivery of fuel. If fuel is missing from any one specific engine cylinder, it will misfire.

Fuel system problems could include some of the following:

  1. Bad fuel injectors.
  2. Broken fuel injector connector (this is a very, very common problem).
  3. Electrical short in the fuel injector wires that are keeping the fuel injector pulse signal from reaching the fuel injector.
    • This is usually the result of human error and after a major mechanical repair where the wiring harness was damaged.
  4. Bad fuel injection computer not pulsing the fuel injector (this is a very rare condition, but it happens).
  5. Bad fuel pump.

Engine Mechanical Condition: The pistons and valves are the ones that draw air into the engine. Usually all cylinders wear out evenly but every now and then, either thru' lack of maintenance or some mechanical problem, you'll have one or more wear out at an accelerated pace.

To make the long story short, those cylinders (with accelerated wear and tear) will produce a less than average compression value that will cause a misfire condition.

Other issues, that can not be overlooked are vacuum leaks.

Honda Vehicles:

  • Accord 2.4L
    • 2003, 2004, 2005, 2006
  • CRV 2.4L
    • 2002, 2003, 2004, 2005, 2006
  • Element 2.4L
    • 2003, 2004, 2005, 2006