My 1996 E300D Check Engine Light with P0120 code came on after using the cruise for a couple of minutes. Anybody knows what is wrong?

Here is what happened. I was back on a long trip. The cruise was on. When I was closed to home, the Check Engine Light came on. At home, I use my Actron scanner to erase the code, drove for another week without using the cruise. No Check Engine Light. Then, I tried and put on the cruise again, LOL, the CEL came back on. Please help.