99 C280 Check engine light keeps coming back on

Hi, I'm new to this forum and a little stressed. I bought a 99 C280 just over a week ago. I love it, except the check engine light has been coming on ever
since I got it. The dealer I got it from has put in an effort to see what's wrong with it. At first we thought it was the gas cap (A common problem after looking at all the threads online) after I learned to close it properly, it came back on again. The dealer had a MB mechanic look at it and he said it was some sort of vacum leak, and tightened it. It was fine for about an hour then the light came back on again, so this time we got a whole new vacum filter. That seemed to fix it, for a whole 3-4 days the light stayed off and I was just starting to relax and enjoy my purchase, when all of a sudden it comes back on again! I'm regretting buying this car, but I'm attached to the luxury of it. The dealer says it's probably just some sensor malfunction, but that won't stop my anxiety until that thing is Off and STAYS OFF. I seem to have bad luck/judgement when buying a reliable used car. I'm a bit jaded and not feeling optimistic. I wanted this car to be RELIABLE along with luxurious. My worst fear is being stalled in the busy L.A traffic that I live, or it not starting when I need it to. I'm a single mother with a small toddler, so i'm a bit paranoid. I'd like to know if this is a common problem that is easily fixed (besides the gas cap thing) or is the MB mechanic missing something. I looked through the previous owners records and noticed he had to have the transmission replaced early on, it now has 94k miles. I'd appreciate your comments. Thanks.
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It would be nice to know what the code is!!
Go back and find out what the fault code is.
Using the Mossberg Technique to fix a car is UNprofessional. [load up the shotgun and throw a bunch of parts at it and hope it fixes it]
Is the 'dealer' you got it from a used car lot??

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I appreciate you taking the time to respond, but I am "new" here I am not a board junkie, so I wasn't prepared with having the code!?! I will get it and post it soon. Yes, the dealer is from a used car lot. And yes, I know it doesn't sound good. Thanks
Karl wrote:

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I agree with Karl... that mechanic should have scanned for code... apparently he does have the tool too reset it so he can read it too. Or, you can go by Autozone and have them pull the code. I suspect you must be in Canada so I wouoldn't know if you have auto store who would freely scan your car.
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