Try a set of NGK ZFR4F-11 plugs. That's what's spec'd for the '97 Civic HX, which uses lean burn. It's not the same plug as the other Civics of that vintage. Your '99 should be the same generation as the '97.
By the same token, you might try using the same primary wide band oxy sensor as that car. A friend has an '04 HX and he says he found the wide band oxy sensor for about $150ish.
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Coast long and prosper.
Driving '00 Honda Insight, acquired Feb 2016.
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