I have a 2000 Civic that’s served me well. I’ve taken good care of it, and thanks to public transportation and a period of unemployment, driven it less than average.
Today, out of the blue, the damned check engine light came on. What are my best and worst case scenarios here?
Best case scenario: Your pollution control doohickey rattled loose.
Worse case scenario: Your car is about to explode. 
In my experience, it usually means there is an error being detected in your emissions control.
Go to an auto parts store, buy some washer fluid (as a courtesy), and ask em to meter the car.
You’ll get a readout (write it down) and some notion of the problem.
BTW–it could be a loose gas cap.
No foolin–those will set off a check engine light.
it just means a fault was detected which may cause the engine’s emissions to rise above what it was certified to. if there’s no driveability change, it’s probably not critical.
if the CEL is flashing, don’t ignore it. that means a repeated misfire and you risk melting down the catalytic converter.
Have you ever had a new catalytic converter put on? How many miles on your car?
Code readers can be had ridiculously cheap. I have one in my pocket right now that I paid less than $10.00 for. It’s paired with my smartphone and I can plug it in, fire up the app and read the codes in just a few seconds.
Possible you didn’t tighten the gas cap enough on your last refill. Happened to me, apparently if some air gets in the tank it can trigger one of those sensors.
Call around to the auto parts stores to see which ones will check the code for free.
Sometimes it’s a problem with the emission control system. Often that won’t affect how the car drives, but you may run into problems if you live in a state which requires an inspection. Sometimes it’s just a sensor which gets gunky and needs to be replaced.
The Car Talk guys used to say just ignore it. If it bothers you, put a piece of masking tape over it. Really.
for that matter, how old is your gas cap? I’ve had that problem, the old and not properly sealing gas cap was setting off the CEL. $65 to diagnose, $10 for generic locking gas cap to replace factory original on a ten year old truck.
The wisest four words I’ve yet heard from either doctors or trusted mechanics about this kind of very generic symptom is “it could be anything”, which indeed it could. The first thing you need to do is get the diagnostic trouble code(s) in the easiest, cheapest way possible for you. Some places might do it free or make a nominal charge, or as mentioned, you can get OBD II readers quite cheaply and they can be very useful over time. Just be aware that when doing it yourself, the DTC codes alone may not have an obvious meaning without some background knowledge which Google may or may not adequately provide.
I once drove for several years with a CEL on due to a failed O2 sensor that wasn’t causing any apparent harm, and the car eventually went to the Great Dealership in Heaven with its engine still in great shape, but it’s always possible that in your case the CEL could be indicating something much more serious and imminent. If everything seems to be running fine, probably not, but don’t risk it!
Damn, it looks like I’ve overpaid for the diagnosis. I was in a hurry, and this was the first time this had ever happened to me, so I panicked a little and just went to the place I usually get my car serviced.
Oh, well, now I know. Just have to wait to see what it ends up being. It better not be the damn gas cap, or I’m going to be really annoyed at myself.
My engine light (2008 Civic) came on Tuesday morning, it was due to the gas attendant not twisting the cap till it clicked. After I “fixed” the glitch, it took 2 days for the light to go off.
Blinking engine light is a misfire, IIRC, which could be timing belt or something else that is horrendously expensive to fix if not immediately acted upon.
ETA: before the SDMB nitpicking starts, the light was on this morning heading into work but off on the way home 
The whole blog post at this link is very informative and thorough. Here’s a snippet.
Take it to an AutoZone or O’Rielly or whatever you got close by. They’ll read the code and tell you what it is. If it was me, anything “sensor” would be put on the “ignore” list.
Of the “newer” cars I have that have Check Engine lights, I’ve put hundreds of thousands of miles on them and never batted an eye. The Nissan tossed a code at 120k. We’re at over 250k now. Damn “fuel temp sensor” or something. Whatever. :rolleyes:
My BMW 740 currently has a FLASHING light. But I know what it is. (misfire detected, most likely cam chain guide failure). Runs like a champ. Just don’t go over 90. I’ll fix it … later.
I guess the poignant answer here for the worst case must be:
all 4 “check engine” lights just came on, and you are 26 miles from Medellin airport.
That happen to me , my check engine light came on so I went an auto shop I never used before and being a woman the guy said he had to run a test that cost
$500 ! I said forget it and remembered about gas cap and when I got home I tighten it and the ‘check engine light’ turned off! I saved my $500 !
Think I read you may need disconnect the battery for a minute to reboot the system for the light to go off.
[QUOTE=Vicsage]
Think I read you may need disconnect the battery for a minute to reboot the system for the light to go off.
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Maybe. Some faults need a “drive cycle” (starting from completely cold and driving until completely warmed up) to clear even after disconnecting the battery. If you disconnect the battery for a moment just before an emissions inspection, hoping to make a fault disappear, some states will give an automatic fail.
If you end up having an O2 sensor problem or something and you are told that you need to put a new cat on your car, FIX THE PROBLEM that caused your cat to take a crap first.
You can get 300,000 miles out of a catalytic converter if your car is running properly (not dumping raw fuel through the exhaust because your car is misfiring, or burning excessive oil because your piston rings are shot.)
If you just put a catalytic converter on a car to get the light to go off, it will soon return and your cat will take a dump on you and you will be in the same boat again…
They usually don’t go out on their own. Something clogs them up (almost most of the time.)
My car is 25 years old with the original cat on it, and its working great. You can test your cat with one of those point the laser type of thermometers. If it is at least 30 or 40 degrees warmer on the tailpipe side of the cat than the engine side, its working.