Why is the light CHECK on?
WHEN HAVING A COMPLETE TUNE UP,THE OXYGEN SENSOR SHOULD BE REPLACED.
It is true, you should replace your O2 sensors when you do a tune up.
My 2000 G20 was purchased used with 162000 miles on it. I drove it for a year with my "Service Engine Soon" light going on and off. Well, my emissions test was coming up and I knew I had to have my car running with no problems. All suggestions I obtained pointed to the O2 sensors needing replacing. My 2000 G20 only has 2 sensors. One in front and one just after the exhaust manifold under the car.
They are not too difficult to change, it just takes a little time. You can do it yourself and save money.
Once you do change them and your service engine light does not go out go to an auto parts house that does free diagnostic and have them " reset " the light to off and drive your car up to 30 minutes. It helps your computer reset as well I am told.
I did this and my light has not been on since.