I changed the oil in my sister's C240 last night. During a road test afterwards, the check engine light comes on during a short full-throttle burst. Now, I don't know if this was caused by the service, or was from a completely unrelated incident. Oil level checks out to be fine by the on-board computer, gas cap is on tight. She has been experiencing a rough idle problem lately, which hasn't thrown trouble codes in the past. Anyone have any experience to share? I'm puzzled...

CEL problems cannot be solved without first getting the codes that threw the light. You need to go from that point to begin a diagnosis. CEL's can be triggered by literally hundreds of things.

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John Shellenberg
1998 C230 "Black Betty" 240K

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Kinda figured that...just hoping that someone might be familiar with common causes based on the sequence of events.

Out of curiosity, what type of system does the OBD use, and where's a good place to pick up a scan tool?

Here you go.
http://www.iequus.com/item.asp?cid=10&pid=3100

That's what I use & you can use it on any 1996 + vehicle, not just M-B.
This will give you " generic " codes, but not " Make specific " codes.

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2007 C 230 Sport.

The most common problem with CEL on the M112 engines is the air mass sensor! Heavy throttle will cause a lean condition to show up. Scan the computer & see if P0 170/173 comes up!

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In addition to the above advice, check for any vacuum lines that may have been disturbed during the oil servicing.