. . year/model and checked your profile; '99 S420.
Could be a bad EGR??? Bad vacuum line?? Bad plug?? One bad spark cable??
With your '99, you don't have as many separate modules (ECUs) as I have so it should be easier to diagnose . . . but not necessarily less expensive.
Even if it's an 'injector' (don't think it is), it could show just at idle and not be noticable at highway speed.
The SRS light is probably the "driver's seat belt module". . normally is. As you said, you need a scanner to reset it. But it will come back!