Im posting this for the next poor sob that has to deal with the god forsaken po300/1399 missfire code.
PO1399 is a Honda specific code it is just the po300 in honda language multiple random miss fires.
If you are reading this Im certain that you have already did a tune up,new cap and wires as well as valve adjustment. Im also certain that you have already replaced the fuel filter and pump and replaced the o2 sensor as well as checked for vacuum leaks. You also checked fuel pressure and your fuel pressure regulator. You have spent a lot of money and Im sure your not happy about it
Now take the idle air control valve off. look inside it, see that little valve in there. Its probably black with dirt. Take that electric motor off the side of the valve and try to move the the valve. If it feels crunchy that is your problem. The IAC is getting stuck either open or closed. causing a lean miss fire. you can now take carb cleaner and clean the valve nice and gently orrrrrrrr get a wire wheel and clean all the crude off that valve and be done with it. Once you did that clean the hole in the throttle body to make sure no crud is in there.. Re assemble everything, clear the codes and put miles on the car. I am willing to bet the codes dont come back.
Good luck
2000 civic ex