The most likely causes would be a leaking intake manifold gasket or possibly a bad spark plug coil. Does it idle rough or vibrate at idle? What are the co and nox readings at? to test a spark plug coil, unplug the wires at the coil packs while the car is running one at a time and see if you have a dead cylinder. if you have one not working at idle, swap it with another coil and see if the dead cylinder moves with the coil pack. if it moves the coil pack is bad, if it stays, the intake manifold gasket may be bad on the intake runner for that cylinder, thats a good starting point.