Engine light 99 e320 wagon also srs light

Got two problems on a 1999 e320 wagon with 100k
first I have the red SRS light going on when there is no passenger in the front seat. Sometimes it goes off and sometimes not. I assume
there must be a faulty switch in the seat. Is this a big job for the do it yourselfer and does the SRS have to be reset.
The other issue is the engine light which started to go on a month or so ago. It is not consistent and i have tightened the gas cap. I notice however that there is not that strong vacum seal when I release it and I wonder if there is such a thing as a faulty gas cap. I am also aware that it could be a faulty sensor. What codes should I expect to see.
Thanx
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dunno if this helps, but this is what happened to one guy......and his response
http://tinyurl.com/2u68el
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