How can i fix the exhaust emissions.?

I have no warning light but my car failed exhaust emissions test , carbon monoxide content After 2nd fast idle excessive (7.3.d.2b? and carbon monoxide content at idle excessive (7.3.d.2c). would it be the oil filter or both oil and air filter , my air filter looks dirty . what els could cause a failed exhaust emissions test and can it be fixed .