Q: Why is my car failing CO levels at idle? It passes everything else with ease.

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My car's emission report shows that it is far below the maximum allowed gas levels except for at idle, where the CO emissions are twice what they should be. I've replaced the spark plug wires, the plugs themselves, air filter, and I've cleaned all the gunk out of the throttle body and intake. Any ideas as to why it won't pass? I realize its an old engine, but it passes every other part of emissions just fine with no issues. Can you give me some ideas as to what I might replace/ clean or fix to get it to pass?

My car has 200000 miles.
My car has a manual transmission.

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