High CO and HC

I have a GM 3.1 V6 powered Isuzu. Every year it passes emissions, but the numbers are close to being out. This year, the CO is 1.03 (Max allowed is
1.20) and the HC is 174 (Max is 220.) This is at idle, loaded the numbers are fine. Last year the numbers were about the same. What would cause this?
Al
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Rich idle mixture is one component.

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