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