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...