I know a number of people have posted claims of white smoke being emitted from their exhaust before, but I am in need of some individual help. About a month ago I bought a '00 Civic Si in good condition, with the CEL on. The owner told me that it was merely due to a bad O2 sensor, and seeing the condition and performance of the car, I believed him. I got the O2 sensor replaced, and the CEL went off as expected. However, after about 15 to 20 miles of driving, the light went back on. I took it to autozone, and they told me the CEL code was P0420, catalyst efficiency low-bank 1. Before I got the O2 sensor replaced, I noticed white smoke coming out of the exhaust, but it was only noticeable at start-up and high-revs, whenever the engine is running cold. The smoke still persists after the O2 sensor has been replaced, while the exhaust emits a unique odor which I am unable to identify, although I know it's definitely not a normal exhaust smell. I have a feeling that coolant is leaking into the engine somewhere, although I am hoping that it is something less expensive to get fixed. Let me know what your opinions are, thanks for your time and help.

 

you said it your self coolant fill up the radiator at night let it sit and check it in the morning if less coolant is found you most likely have a head gastket it will also cause your cat to go bad and your o2 air fuel sensor

 

Hmm, most cars have white smoke when they are cold ... Check your anti-freeze levels and if they are going down it could be waterpump(check for leaks under your car) head gasket .. Radiator in bad shape ? ... Did you get a pressure test done yet ?