P0420 Arrrgh!

I think I now now why I cant get rid of this code. I have had the catalytic converter replaced this month (magnaflow), the rear o2 sensor was replaced today. I replaced the front O2 sensor 3 months ago. Each time i got the code erased, and then it comes back. This last o2 sensor install, the O2 sensor was white and the slits were caked with the unknown white substance. I put in a new one and had the code removed, yet for it to come back again. I also am constantly topping off the coolant bottle.

I replaced the front three spark plugs the other day and I am going to replace the rear this weekend. Could the white cakey stuff be dexcool in the exhaust via a sparkplug port? I dont get any smoke out of the exhaust and I was getting pretty poor gas mileage. I am driving to Oklahoma the end of this week and I was hoping to get the best mileage I could. Any ideas?

Welcome to AF. Coolant in the exhaust can damage a cat, find the leak either way. P0420 can be a complex diagnosis as you see. Do a site search for this, lots of info.