do both sensors need spacers? cause i got two on the way. my car is a 2002The real solution is fixing the 2nd o2 sensor and if you do get an aftermarket header, you'll need to insert a spacer that you screw into the o2 bung then insert the sensor on top of that, that avoids triggering the P0420 again. Purpose is to keep the sensor from sniffing the exhaust, sometimes double stacking spacers is necessary. The o2 spacer technique is not really recommended if your car is a 2006.