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?

by in Lemoore, CA on December 29, 2010
2 answers 3 comments
ANSWER by on December 29, 2010
well, you need compression, spark and fuel to make the cylinder work. all these things need to be checked. Roy
COMMENT by on December 29, 2010
what about oxygen?
COMMENT by on January 24, 2011
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.
ANSWER by on December 29, 2010
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.
COMMENT by on January 24, 2011
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.
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