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Discussion Starter #1
Its kind of an advertisement for Autotap but here it is anyway: Link to article.
The Check Engine Light, also called Service Engine Soon (SES) or Malfunction Indicator Lamp (MIL) on OBDII equipped cars really has a mind of its own. You can be sure that it never behaves quite the way you think it should!

Let me start by clearing up a common myth, the check engine light's purpose is not to warn you that your engine's malfunctioning. In fact, the light's true intent is to indicate when the pollution-control equipment on your car is malfunctioning.

Don't get too relaxed, the vast majority of check-engine lights still mean that something is going wrong with your car, and it may very well be something that could leave you stranded beside the highway if you don't take care of it.

There is some good news behind this check engine light. More on that in a minute. First let's look at what makes it turn on and off.

Depending on the type of failure detected by the computer, one of three things can happen: The light may turn on immediately, a "pending" code may be set (which waits until the condition repeats before turning on the light), or the light begins to flash.

Flashing Light - If your check engine light starts flashing, it's telling you that you've got a problem that could cause further damage to your vehicle if you were to keep driving.

Pending Code - A bit tricky because it's invisible to anyone that doesn't own AutoTap or a similar scantool. Pending codes go away automatically if the computer is unable to detect the failure condition a second or third time. If the condition is detected again the pending code turns into a real code (technically called Historical) and your Check Engine light turns on. Periodically checking for pending codes (particularly on higher mileage OBDII cars) can be revealing since they can often tell you of upcoming problems long before they cause a detectible symptom. Here's another tip - if you're shopping for a used car check, use AutoTap to check the car for pending codes before you buy.

Continuously on light - This one means that a problem has been detected. Sometimes you'll be able to detect a drivability problem, sometimes not.

So what's the good news? Anytime the light turns on, a Diagnostic Trouble Code (DTC) is set and Freeze Frame data is recorded. What does that mean to you? Simply use AutoTap or another scantool to read the DTC, which will give you a basic description of the fault. You can also read the Freeze Frame information which is snapshot of conditions when the code was set (invaluable for intermittent problems). The DTC system is remarkably detailed and provides a tremendous amount of guidance for the DIYer.

Turning off the light - If the computer doesn't detect the fault condition again for a number of drive cycles (drive cycles are an interesting topic, lets make that a topic for another article) the light will turn itself off.

While it's a happy day for the DIYer when the problem goes away by itself, don't hold your breath. The right thing to do is use your AutoTap or other scantool to read the code, then use the real-time data features to apply some common sense to what the computer's reporting. From that point, you can often pinpoint the problem before you've even popped the hood. Once you fix the problem, use the Clear feature of AutoTap to turn off the light and you're done.

The final brute-force method is to disconnect the battery. Obviously this doesn't fix anything, but it will make the light turn off for one or two drive cycles. The problem is you also lose radio settings and the computer loses its tuning information, which can take many drive cycles to "relearn".
How close is this to the truth? Is it even generally correct? I can't imagine its the same for all OBDII cars but is it?

The reason I ask is that I read a post that says the computer goes into "Closed Loop" mode after a CEL has been thrown. But I cannot understand why.
 

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Bump.
 

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Discussion Starter #3
C'mon, there's gotta be somebody with a comment on this?
 

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I don't totally agree with this part

Let me start by clearing up a common myth, the check engine light's purpose is not to warn you that your engine's malfunctioning. In fact, the light's true intent is to indicate when the pollution-control equipment on your car is malfunctioning
but the rest is true. Look at the list of DTC's in the sticky at the top of this forum to see what the MIL will tell you.

I got a scanner from McNew's and I love it. I have used it to do some diagnosis on my car, find the cause of a MIL on my sister-in-law's 1999 Buick LeSabre, and on her husband's 1996 F-150.

There is a lot of information available at the OBD II port not just the cause of a MIL. My scanner has a real time data mode that I use to monitor things like fuel pressure, timing advance, and MAF flow as I am driving. This allows me to see what affect changes I have made are having.
 
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