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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.

Hi and thanks for contacting YourMechanic. You may have to replace the distributor cap and rotor for a good, strong spark. Then, get some sea foam and put it in the vacuum system of the engine. This will clean the valves off and help the rings seal better for a cleaner burn.

Change the engine oil and filter for a clean engine. When you take the vehicle to the emissions test, drive the vehicle and get the engine hot like on the freeway, and then keep the engine running until you have to shut it off at the emission station for the test. This will help keep the exhaust hot which will keep the emissions low.

If you need further assistance with your vehicle, then seek out a professional mechanic, such as one from Your Mechanic, to help you make sure your exhaust system is in top condition.

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