So what is the deal with oxygen sensors on my 2004 Honda Civic DX?
 

Hi,















My primary oxygen sensor has not been working for several months now (I got the diagnostic but never replaced the part). I want to replace it but I've heard its best to replace both at the same time. Is the primary o2 sensor the one that I can see when I pop the hood--right at the front of the engine? Where is the second one? I know it is near the catalytic converter but I do not know where that is. I'd really rather just spend the money on the primary but if buying both is a much better idea I'll do it... thank you!

Do both. By just opening your hood I am imagining you are seeing the one on the exhaust manifold. The other one that you need to change should be near your C Converter. To find this just follow your exhaust tubing from the manifold underneath the car to where it widens out and sorta looks like a flat muffler. It should be in the middle of the car area. That is your C Converter. Oxy sensor should be right there. That is the where, as for the how it is kinda self explanatory. Take off old sensors install new. For any questions or if you plan on doing a lot of little repairs to your car get a haynes manual. Should give you step by step directions on just about any repair.