The o2 sensor is not heated so it don't work real well with headers. Headers have TONS of surface area and are very thin compared to the factory manifolds so by the time the exhaust reaches an o2 sensor in the collector it is not hot enough to work real good. Without a properly working o2 you risk running lean as it will revert back to the base fuel map and the free flowing exhaust is now helping more air come in the engine and no way to add more fuel. If you have to have headers get good ceramic coated ones.