Unplug the o2 sensor, adjust the throttle stop on the throttle body to get the car to idle 8-900 rpm. Check the tps for adjustment, it should go open about 200rpm above idle. Make sure all the idle and emission controls are disconnected as they won't work with the z31 ecu, and that all vac lines that have been disconnected are plugged. Erase the cas code and see if it pops up again.