service engine soon light

Hello all.....I just bought a 2001 alero a week ago. Last night the service engine soon light came on as i was driving home. I took it to my local garage ( and a mechanic i do trust ), and he plugged in his computer. I didn't see the code number, but the warning showed that the water temp was only operating at 170 degrees when the light came on. I also noticed that fuel economy on this 2nd tank was much lower than the 1st. I guess my question is, could the thermostat not working properly make the light come on and effect fuel economy that much? Or is there some type of emission problem that didn't show a code. Thanks in advance...:)

What kind of scanner was he/she using? The $100.00 scanner are cheep, they do work to an extent, but not the best. On the 15 grand cpu's that the dealer uses and other co. use, they are the best and will show all codes. as far as the question goes, you can replace the thermostat, and see if it happens. i think you would have to have the code removed by the dealer or so, and wait to see if you correcd the problem.

Not sure what kind of scanner he has. They replaced the thermostat today and cleared the code. So far so good, but the car hasn't really been driven much yet. So I'm hoping this was the problem.