93 accord check engine light

hey,My 93 10th anniversary accord's check engine light keeps coming on.it seems to come on after about 30 minutes of driving,it comes on faster in stop and go traffic,and slower if my heater is on. The previous owner had a diagnostic done at the dealer but they couldn't find anything. I'd like to find out what it could be before i let the dealer rip it apart.also the steering wheel seems to vibrate more than it should when the light is on and if the light comes on while i am stopped i notice that the brake pedal goes down about an inch, thanks for your help, mike
ps. 96,000 mi.