Check Engine Light 1994 E320

[sfeyrer]

I did a 75000 mile service on my 1994 E320 last weekend.
- transmission service
- oil filter and change
- fuel filter
- air filter
- changed brake fluid and front pads
that was on Saturday, on Monday my check engine light came on. I checked the fuel door and it was locked, so I fixed the problem and turned the check engine light off by disconnecting the battery. Yesterday the light came back on and the fuel door is fine. The light conttinues to stay on even after the car is turned off. Any idea's? I just bought the car w\ 73000 miles on it. The car seems to run great even w\ the light on. Thank you for any help.