Check engine light illuminates on 2007 Saturn Ion

The check engine light of my 2007 Saturn Ion 3 illuminates constantly. I took it to a repair shop and the owner told me that the 3rd cylinder is misfiring. However, no one could figure out the cause of the problem I took my car to a friend's house, and he used a code reader to extract the trouble code from the computer. I don't know what the code was, but he confirmed a misfire in the 3rd cylinder. The car seems to run fine, but the check engine light stays on. What could the problem be?

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2 answers 3 comments
well, you need compression, spark and fuel to make the cylinder work. all these things need to be checked.

Roy
what about oxygen?
I took my car to another mechanic, and he performed a compression check on all 4 cylinders. He discovered that the compression was relatively low in the 3rd cylinder compared to the others. He believes there is a problem in the valves. I hope he is right, because a valve job is pretty expensive.
A professional grade scan tool will give misfire count per cylinder, and air/fuel ratio fuel trim figures to show if a cylinder is "lean", as Roy said, check compression see its even, look for vacuum leaks at manifold, mis-fires can be hard to trace without training or very good scan tool.
I took my car to another mechanic, and he performed a compression check on all 4 cylinders. He discovered that the compression was relatively low in the 3rd cylinder compared to the others. He believes there is a problem in the valves. I hope he is right, because a valve job is pretty expensive.