I've had a "Service engine soon" light come on my 03 Chevy Trailblazer on and off for nearly two years. My mechanic looked into it a couple times and just advised me to ignore it. The category the warning fell into amounted to I was possibly polluting a little more than spec. This happens on and off every couple months fo rover two years and after a couple days will go off. A couple weeks ago it came on again and stayed on steady. Since I was ready for an oil change anyway, I had him look into it and he did find where the engine temp wasn't coming up to normal. I had actually noticed that on the temp gauge, that it had been running cool, below 180, for awhile. Turns out the thermostat needs replacing. Had that done and it does make a difference, even seems to run smoother. Time will tell if the little light stays off. I guess after 81,000 miles it is due for a few minor updates. He checked it over closely as I am going to go to CO and AZ in a couple months and he gave a good bill of health on everything else. A good trustworthy mechanic is as valuable as a good trustworthy doctor.