How To Test Misfire Codes (Honda 1.5L)

Quite a few things can cause a misfire in one or several cylinders. For example: bad spark plugs, a bad distributor cap, bad spark plug wires, low engine compression in one or more cylinders, bad fuel injector, etc.

I could go on and on, listing more possible ‘bad’ components but you get the idea. The good news is that with a good diagnostic strategy (of tests) you can find out exactly what's wrong yourself.

In this tutorial, I'm going to explain in some detail the most common causes of misfires 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 or rough idle condition your Honda is experiencing.

Let's get started by jumping right into the next subheading...

En Español You can find this tutorial in Spanish here: Cómo Probar Una Falla En Cilindro (1.5L Honda) (at: autotecnico-online.com).

What Is A Misfire Condition?

Every cylinder, in your 1.5L Honda Civic's 4 cylinder engine needs air (compression), fuel, and spark to produce power.

It's when one of these 3 things is missing from a cylinder that that cylinder will misfire. This misfire will cause a rough idle condition and/or a misfire on acceleration.

Here are a few other symptoms you'll see when your 1.5L Honda Civic is misfiring:

  1. If OBD II equipped, the Check Engine Light will be on with one or more of the following misfire codes:
    1. P0300 Random Cylinder Misfire.
    2. P0301 Cylinder #1 Misfire.
    3. P0302 Cylinder #2 Misfire.
    4. P0303 Cylinder #3 Misfire.
    5. P0304 Cylinder #4 Misfire.
  2. Lack of power upon acceleration.
  3. Smell of unburned gas exiting the tail pipe.
  4. Rough idle and may stall.
  5. Cranks but does not start.
  6. Will not pass the emissions tests.
  7. 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?

Keeping in mind that each cylinder in your 1.5L Honda Civic's engine needs spark, fuel, and air (compression), we can start looking at the specific systems that are tasked with providing them.

Ignition System: The ignition system is tasked with creating and delivering spark to each of the 4 cylinders. The Ignition System is usually the culprit behind most misfires.

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 distributor cap.
  4. Oil dripping (from the valve cover) onto the spark plugs and spark plug boots.

Fuel System: The fuel system is responsible for making sure that fuel reaches each cylinder. Fuel system components that can cause a cylinder misfire, when they fail, are:

  1. BAD fuel injectors (multi-port fuel injected Civics).
  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.
    1. 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 cylinder head valves are the ones that draw air into the engine and compress it. 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) produce a less than average compression value that will cause a misfire condition.

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

With this info under our belts, let's turn the page and find out what to test.