I have had my B18C swap in for a little over a year now and I have noticed that I am getting some blueish black smoke out of my exhaust at high rpm (above 5000) I do not have a cat installed could this be the cause of it or could it be valve seals. I did a compression test last weekend and the cylinders were 245psi across the board so i think my rings are ok. I am also getting an o2 sensor code could this be because i am not using a cat. Once the engine is warmed up and you shut the car off and turn it back on the check light goes away. Thanks for any info you can give me

 

With those compression numbers, cylinder sealing sounds just fine. Valve seals it is then.

I'm assuming it's only using one oxygen sensor. The o2 sensor code isn't from not using a cat. The sensor is before the cat, and the cat shouldn't have any bearing on the health of the sensor. I say pull it out (22mm open-end wrench) and look at it. Gently clean it with a tooth brush and some carb cleaner if it looks fouled/caked. Replace if necessary. Check all wiring continuity with wiggling regardless.