1988 Mercedes Benz 300e Engine Check Light

Electrical problem
1988 Mercedes Benz 300e 6 cyl Two Wheel Drive Automatic

My MB, which has been very reliable, has had the Engine check light come on, and will stay on after I turn the engine off, which has drained the battery. I only use the car on the weekends, as I am in my rig during the week. A Mechanic determined the light is coming on because of Code 7, which he will not tell me is because of(?), I'm sure he wants a large ransom(?) Not. I have figured it is a Rich/lean problem, which may be as simple as a Tune-up, to the Injectors (Ouch!), both of which are a DIY My MB has over 230,000 miles, so I would figure any major repair would be pointless at this stage. Though I do want to say, that this is the first trouble I have had with my Baby. I would like to keep her! Can someone tell me how I can turn off the light so it will not come on after turning off the Engine? Thanks!