I have a 2002 Subaru Outback with 45,000 miles, and in good condition. Suddenly the “Check Engine” light came on. Manual says “Go to dealer immediately to prevent serious damage, may be failure of emission control.” But, recently I passed the emission test with flying colors. What do they mean by serious damage and damage to what? Subaru’s net profit 
or there will be real damage to the car? How concerned should I be?
If the light is not flashing then no imminent destruction awaits. The “drama” present in the owner’s manual is about maintaining the Subaru dealer’s profit, but the message that triggers the light is very real. It means that the car’s computer has detected a problem with one or more of the car’s systems.
If you got gas recently, the first thing you can do is check your gas cap. Make sure it is still there and that it was put back on correctly. If the gas cap isn’t sealed it will produce a code.
You can go to most any major auto parts store and ask them to read your codes. Many do it for free. Then you can post the codes here for advice. The typical code is a P followed by 4 digits (e.g. P0123) - get the actual code, not what anyone tells you about it.
If you don’t want to fuss with it at all, you just need a good reputable local mechanic - you don’t need a dealer.
Thank you for the fast reply! That’s good to hear. I checked the gas cap and it is fine. Can you recommend an auto parts store?
That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.
Regarding warning lights:
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if the coolant temp light comes on, shut off the engine ASAP
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if the oil warning light comes on, shut off the engine ASAP
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if a FLASHING MIL/CEL comes on, shut off the engine ASAP
ASAP means driving to the berm of the highway right now and not waiting for the next exit.
But if the MIL/CEL is not flashing, then it’s not an urgent indicator.
In my area, Auto Zone and Advance Auto read the codes for free. O’Reilly loans you the meter to use in the parking lot. I go to Advance Auto, usually. There does not appear to be a nationwide policy on this, from what I have read, so your experience may vary.
I have AutoZone in my area- I will check it out there.
Thanks so much for the help!
Thank you very much! I really apreciate your help! Pep Boys have read my code for free! It is P0038 (Rear oxygen sensor heater circuit high input). How bad is it? 
You got to measure it with a multimeter and find out how bad it is. The fault could be in the oxygen sensor, or in its wiring.
You can find the location of the rear oxygen sensor which is in the exhaust from #1 cylinder side: http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?chapterTitle=Component+Locations&partName=Engine+Performance+%26+Emission+Controls&pageId=0900c1528019d228&subChapterTitle=Heated+Oxygen+Sensor++H02s&partId=0900c1528019d1c8
You may have to register to see the pictures; but, that’s not hard.
Let’s assume the Oxygen sensor totally failed or is dead - can I drive my car for another 6 months without worrying about it? And, since it is the rear sensor, which means it’s at the end of the exhaust line, what damage can happen to the car, in the worst case scenario?
Thanks for the help!
If the car emmissions did not need a rear sensor they would not place one there. The readings from the rear sensor affect the entire upstream.
My wife ignored her rear sensor code(intermitent though) on her 140k mile 96 Civic for about a year. It eventually ruined the upstream sensor and the cat converter. Finally it started to stall at highway speeds slowing down.
She took it to dealer(fortunately in this case) and both sensors + cat converter were covered luckily in some sort of Honda warranty extended for whatever reason. The bill of dealer otherwise would have totaled about $1600.
Right now, the heater circuit isn’t working properly on that rear oxygen sensor. The rear oxygen sensor function will be delayed until the exhaust heats it to operating temperature.
The drawback to not getting the check engine light to go off is that if something else happens, the light can’t tell you of it. Why? Because it’s already on!
The problem is that the rear sensor is used to determine how effectively the catalytic converter is removing oxygen from the exhaust stream. Odds are that the vehicle computer – believing (likely correctly) that the rear O2 sensor is dead – will simply stop trying to dynamically adjust the fuel mixture and will fall back to some predetermined – one size fits all – mixing mode. That might work fine. Or it might affect your gas mileage. Or maybe it will eventually damage something, Maybe it will destroy the catalytic converter if you keep on running that way. I don’t think there is any way to tell which.
Replacing an oxygen sensor isn’t usually a gazillion dollar repair and it doesn’t usually involve disassembling half the car to get to the O2 sensor. Why don’t you get a quote from an independent mechanic for installing an after market O2 sensor. It may not be all that frightening. If you have ramps and tools, you might even consider replacing it yourself although getting the old sensor out isn’t always easy.