I just replaced my O2 sensor on a 96 Civic. I don't know how much different it is for a 99. It wasn't too difficult, but definitely need patience. I purchased the new one from the local automotive store and also "rented" the socket set, too. You will definitely need the socket set. Their policy is you pay for it when you "rent" it (about $30) and then when you bring it back, you get your money back. I used an OEM version and it cost about $80. Your car will still run if the O2 sensor is bad, but the engine light will remain on and then you chance something else happening and not knowing. It's worth the investment.