Chrysler Repair: service engine soon light, chrysler town and country, mechanic check

Question
What does "service engine soon" light really mean? My son drove his Saturn quite a while with it on and the dealer couln't find anything wrong. Now we just bought an older mini van ('96)Chrysler, Town and Country and that night this light came on.Before we bought it we had a mechanic check out the van carefully for any signs that it might not run well, and he (and we) didn't see anything. The van looked like it had been well maintained.  Thanks for your advice.

Answer
Hi Cindy,
I'm not familiar with the Saturn* so I can't say for sure, but the "check engine" light on the Chrysler means that the engine electronic controller has observed a problem and has stored it as a coded number in its memory. Fortunately you may not have to go to a dealer, so let's give it a try:
The most useful thing to do would be to try to get the fault codes that may stored in the engine controller memory to readout. Try using the ignition key: turn it "on-off-on-off-on" and leave it "on" (doing this quickly, no longer than 5 seconds). By "on" I mean just the normal position when the engine is running, not the cranking position. The 'check engine'light will remain 'on' when you leave the key in the "on" position with the engine still not running. But then watch the 'check engine' light to begin flashing, then pause, flashing, pause, etc. Count the number of flashes before each pause and keep track of the numbers. Repeat the readout and verify the counts are correct. Then group them in pairs in the order that they came out, thus forming two digit numbers. You may notice that the pause is shorter between the digits of a given number, and longer between the numbers themselves. Then send me a 'follow-up' question telling me the results of your readout. By the way, 55 will be the last number (two groups of 5 flashes each) and that is the code for "end of readout".
I have the troubleshooting manual for several engines and we can look up the possibilities of what is wrong based upon what fault codes you show. When you write back tell me what size engine (how many 'L''s engraved, or its listed on the underhood sticker) you are driving.
There is also an essay on fault codes at the site:
http://www.allpar.com/fix/codes.html
which gives the meaning of the code numbers. But then you need to get specific info for what exactly might be the diagnostic tests or parts to replace to complete the repair. If this approach doesn't produce fault codes at all, not even a 55, then your engine controller may be too modern to let you get by with this approach and you will have to go to a good tune-up shop where they have a more sophisticated code reader in order see if there are any codes. You should be able to get a readout for under $50.
Still, right me back and we'll talk about what is the best way to get it solved. But I think you may be able to get the check engine light to "tell its tale".
Roland
*I believe it too means the controller has noticed a problem but I don't know if you can read out the memory yourself on the Saturn.