Check engine light

I just bought a 1999 Miata. The check engine light came on shortly after I bought it. I determined the that the gas cap was bad and
replaced it. The light did not go off immediately, but went off about a day later and stayed off. The dealer told me that this is typical. He said that the computer will go out and do a diagnostic every so often and report what it finds. If the gas cap was fixed, the computer would turn out the light.
Does anyone know the actual function of the check engine light and what the frequency of these checks are? I recently replaced the battery and when I started driving again the check engine light came on. Is it typical for the light to come on when the battery is disconnected for a while? How long do I need to wait to see if it will go out and stay out?
Thanks for your help.
Brian Turner
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Brian Turner wrote:

The "check engine" light indicates "a problem", usually with excess emissions. Most of the items the computer responds to are EPA-mandated.
There is no way the owner can "read" the error codes without fairly expensive equipment or a trip to your service facility. Even then, its not unusual that the computer reports no malfunction.
The connection between the "check engine" light and the loose/bad gas cap is the unsealed filler allows hydrocarbon fumes into the atmosphere; that's a government-mandated alarm. I've done this two or three times with my car; the light is usually gone the next day
If its on steadily, the car can be used and checked later; if it is blinking, stop and figure things out right away. Generally, any check engine light which goes out on its own indicates a transient problem that requires no repair. Its reasonable to wait 1-2 days, or a few engine on/off cycles, as long as the car feels and sounds normal.
Joe Silver '99
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The check engine light can turn on when any one of several continuous tests fail, or as a result of the federal mandated test sequence failing. You would need to look at a shop manual to see the complexities of the whole thing. A flashing ecu light indicates some sort of catastrophic failure, and is warning you to not drive the car until the problem is identified and fixed.

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Hi Brian,
Congratulations on your purchase!
When I got my '99 (used) the light came on and eventually required an $800 repair - covered by the warranty. The dealer tried to weasel out of it by suggesting it was the loose gas cap, or moisture from detailing.
Since then I've had it come on for a loose gas cap a couple of times. After tightening the cap the light went off after a day or two.
At my first inspection with the new regulations (in NJ) the light was on and the car didn't pass. I had the inspection done by my mechanic and he explained what the fail code probably indicated. I looked this up on Miata.net and found the fix was fairly simple - clean a dirty/clogged egr valve I think it was. I did it and the car passed.
What I learned from my mechanic was that you can have several fail codes that all come about because one thing is wrong. Fix the one thing and all the others "may" go away. Also, the system has to go through what he called "drive cycles" before the fail codes are cleared. The fact that the light has gone off doesn't necessarily mean the internal fail code is gone. This is good to keep in mind if you do the repair/maintenance and the light goes off and you then return to have your car reinspected and it still fails and your mechanic thinks you've just tried to fool everybody by disconnecting the battery to get the light off. I wasn't foolish enough to do this of course.
Since then, I purchased an OBDII scanner that plugs into my laptop. It's very simple to use and reports the error codes. Without this information, you are totally at the mercy of whoever works on your car. In my case, I trusted my guy a reasonable amount, but he only could read the codes by using the inspection computer. The local Miata dealer would have been my other possibility. The cost of the scanner was somewhat less than the at least $200 I always have to pay any time one of my car's is in the shop - I shudder to think what the dealer would soak me for.
One of the parts franchises apparently reads codes for free - when I called though there were all sorts of obstacles.
I don't know the accuracy or reliability of the OBD scanners, but I haven't heard anything bad so far. They vary in cost depending on features and whether you need a computer to attach to. Still, I think worth it to know.
Gary Fuchs '99
Brian Turner wrote:

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