Something that I haven't seen anyone mention on this thread is the catalytic converter. When I took my car for its first emissions test it failed both HC and CO. I had already taken all the normal precautions including new plugs, wires, cap, rotor, oil, air filter, check for vacuum leaks, etc. I also made sure the car was at full operating temperature by running it on the highway for a while before going to the test center. It still failed. And I DID try a "Guaranteed to Pass" additive. Didn't help. What I discovered later was that my catalytic converter was EMPTY. There was just nothing in it. Now, my car was previously owned by a cop so...read that whichever way you want. I replaced the catalytic converter and the car passed with flying colors. The cats are a little expensive but very easy to put on. I did it in about 2 hours (including time to clean the engine off first). --Rob