I have the same issues. Cylinder #1 and Multiple misfires and CAT errors.
took it was while a back to my local mechanic and he first replaced Spark Plugs...nothing
Then he suggested that we replace CAT and Engine Manifold. Did that, still nothing...car still shakes and SES light keeps blinking/flashing. Rough starts and it seems like something is "clogged" somewhere. Thats what it feels like when I am driving. After driving for a while the car seems to get a jolt of power and runs normal, most times, the flashing/rough ride starts again.
Going to try to replace Head Gasket and Fuel Injector, rather than buying a new motor for now. If that doesnt work, then will sell car for parts. I wont spend $1800 to $2000 on a 2003 Altima with 205K KM's on it.
Fed up with this car.