How to Trouble-Shoot a Check Engine Light - Corvette Forum : Corvette Forums
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Old 09-07-2010, 01:48 PM   #1
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How to Trouble-Shoot a Check Engine Light

If you have not experienced it yet, one day you will. You will go to start the car or you may be enroute your destination when it happens:

So there you are, trying not to panic. The thing is, that sucker is annoying! It's like having a piece of lettuce stuck in your teeth on a blind date with Daisy Fuentes. It feels like it depreciates your Vette's value by at least $10,000. It screams, "I am a piece of crap!"

Well, never fear boys and girls, the Junkman is gonna help you work it out. Here's what you need to do.

1. Pull the codes! This is such a no brainer and yet, I'll see someone start a thread with the question, "Hey, my check engine light came on. What do you all think the problem is?" Answer... we don't have a clue without knowing what code is illuminating the MIL, or malfunction indicator lamp as it is technically called. How do you pull the codes? See step 2.

2. To pull the codes, turn the ignition key to the on position but DO NOT start the car. Make sure that your doors and trunk are closed so that the DIC (digital information center) doesn't start nagging you about a bunch of stuff.

Next, hit the reset button to clear any nag messages that may be on the DIC.

Then, while holding the Options button down, press the fuel button 4 times within a 10 second period.

3. At that point, the DIC will automatically start displaying all of the various modules and any codes that may be present in each module. You can stop this automatic mode and send the DIC into manual mode by pressing any button except the E/M button (that button causes the DIC to exit the diagnostic mode). You can then scroll through the codes using the Gauges button to go forward, and the Fuel button to go backwards. Here's an example of a code notification problem:

As you can see by the picture above, the Power Control Module (PCM) is displaying that it contains 2 codes. Because I have the service manual for my car, I know that the PCM is designed to maintain exhaust emission levels while maintaining excellent driveability and fuel efficiency. I also know that the PCM controls the following operations:

Fuel control
Ignition Control (IC)
Knock Sensor (KS) system
Automatic transmission shift functions
Manual Transmission: 1-4 Upshift and Reverse Inhibit
Cruise Control Enable (if so equipped)
Evaporative Emission (EVAP) Purge
A/C Clutch Control
Cooling Fan Control
Secondary Air Injection (AIR)

Armed with this information, I now have a clue as to what area of the car to start looking at, but there's more. While in the manual mode I can look at the actual codes that exist. By hitting the Options button again while the DIC is displaying the screen above, it will show me the actual codes. Doing so resulted in the following:

The first picture shows a PCM code of P0300 H. When writing this information down (which is what you should be doing while looking at it), you MUST be sure and include the module (PCM in this case), the code (P0300) and whether or not the set code is history (H) or current (C). This is detrimental information in trouble-shooting your codes. So many times, people will just write down the numbers. You need to write down everything.

The first picture is showing a code that is history, meaning it happened in the past and is no longer a issue.

The second picture is another story. It shows a code (PCM P0440 H C) that is not only history, but is also current. That means the situation existed in the past and is now a present issue. As it turns out, when this situation happens, it will illuminate the check engine light (or MIL).

The rest of my modules contained no codes so as I scrolled through them, they all read "no codes".

Now that I am armed with a detailed error code, I can check it against a data base of all the codes that can be possibly thrown by my car. That data base is located here. Locate the year of your car and then find the code that is being thrown. That will give you a basic description of the code being thrown. However, if you have the service manual for your car, you can look up the code and get a more thorough description of the code being thrown. Here's an example using the code that I was experiencing of what you will find in the service manual. Advantage: service manual.

4. At this point, I can do a detailed check into why my car threw this code. As you can see by looking at the page with my code, it even walks me through how to resolve my issue. The description is thorough and detailed. With the right tools and the service manual, I can save myself thousands of dollars in repairs. This is why no one touches the Junkman's ride but the Junkman. I have armed myself with the tools to remedy my own issues.

5. After you pull your codes and have wrote them down, you can then clear them. Any code that sets the MIL must be cleared before the MIL will go out. In order to clear a code, you must have it displayed first and then, hold the reset button down until the code clears. It will disappear and you will hear a beep. Again, you can scroll through the codes by pressing either the Gages button to go forward, or the Fuel button to go back.

Some codes a just a fluke. Sometimes, I tell people to reset all of their codes and see if they come back. If they immediately return, then you have an issue. If it returns a year later, then it was obviously a fluke. Still, you need to keep an eye on these things because not all codes will illuminate the MIL.

Right now my cluster looks like this:

No more screaming check engine light! My car has just increased in value by $10,000! The sun is shining and Daisy wants me bad!

Yea, I wish.

The Junkman

P.S. Say hi to Daisy...

Last edited by Junkman2008; 10-11-2012 at 08:26 AM.
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