97 e420 rough idle issue

I have a 97 e420 that has a check engine electronics light on and is running like crap.. i.e. rough idle ...sluggish acceleration. the day it happened was an extremely hot day outside temps were 100+ and was in moderate traffic..a/c running..

Don't want to have to pay major bucks if we can replace a faulty part ourselves.. any suggestions and help would be most appreciated.