Engine Performance problem
2002 Other Saturn Models 4 cyl Front Wheel Drive Manual 95,000 miles

My "service engine soon" light keeps coming on. I recently had a complete tune-up on my car, new plugs, new plug wires, new air filter, and oil change and filter and this did not correct the problem. The mechanic then told me there was a carbon build-up on cylinder 1, said he cleaned it and the light went off for a couple of weeks. Now the light is back on, however, the engine appears to be running and idling just fine. What do I check next??