Pontiac trans sport 92. 3,8 code44

I change 4 oxygen sensor and still dont work. CO to high.

Is it possible that I dont have exhaust manifold gasket and oxygen sucked in. oxygen sensor dont show voltage ???

maybe your air fuel ratio is to lean ??

Now I know that PCM was broken. I change it and CO was ok. Only my gearbox dont work. When oil is warm then 4th gear dont work.