I have been having an issue with the Service Engine Soon light coming on completely randomly on my 91 K1500. It only happens at highway speed but with no rhyme or reason. I will be traveling at highway speed and it will come on, then with no change in speed or terrain it will go off. Anywhere from 15 to 45 minutes on then anywhere from 15 minutes to an hour later it will come back on. I have recently had the oil changed, tranny checked (OK per tranny shop) and it has a fresh tune up in December. Nothing changes in the way the truck acts when the light is on or off. Is this something to be concerned about? I know that it says service engine soon so it isn't like a warning to get the thing off the road right now but it makes me nervous to have any dash light on. It would make more sense to me if it came on and stayed on but this intermittent deal is making me really nervous.