My truck is failing smog check due to high CO, what might cause this?

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My truck has passed with flying colors on everything by the CO, it is about twice the limit for CO. The vacumm hose and O2 sensors have been replaced, to very little effect. The truck runs great, the tail pipe is clean, and there are no diagnostic codes in the computer. finally got the Frontier to pass smog. Pulled the plugs and cleaned the piston tops, flushed all the fuel lines. After this it finally had the CO come down to acceptable levels.