Backfiring 2000 Nissan Quest

why does my car backfire and hard to start

I really suggest that you get this vehicle to a qualified mechanic a.s.a.p., before permanent engine damage takes place, but if you want some clues to begin the process of elimination, there could be a problem with the fuel/air mixture, or the ignition timing. However, those problems are rare in a modern vehicle that has been well-maintained. Other possibilities include bad spark plugs and damaged spark plug wires.

How well has this vehicle been maintained?
How long has the Check Engine Light been lit-up?
Is the CEL lit steadily, or is it flashing/blinking?