I'm working on a 2002 Mazda protege with about 185,000 miles on it. It smokes extremely bad on start-up. After it warms up, the smoke seems to go away but the car generally needs to be driven for a bit before the smoke stops. I thought it was the valve seals so I pulled the head and rebuilt it but it still smokes.
Had a friend look at it and he said it's also running rich judging by the smoke. He said the gas could be washing the cylinder walls and allowing it to burn oil. The O2 sensor was replaced at some point but I'm not sure when that was. I can only tell because it has a universal O2 sensor installed and he said he'd replace that with an OEM since the universal isn't calibrated specifically for the Protege. There is no check engine light and I attached a scanner and there were no codes stored.
How likely is it that I need an O2 sensor? What else could be the problem? Is it likely the rings are shot?