Can someone explain error code p1399. Is there a specific problem for this year accord?

 

That is the random misfire code.

If it is during idle the plugs may be loose, worn, or other ignition issues, spark plug wires or coil(s) depending on engine.
If it is during part throttle cruise, most likely a lean misfire caused by clogged EGR passage(s) in the intake manifold.

 

I cleand all the ports in the manifold. Bought a new egr valve and replaced all the spark plugs. New wires also. Evap system?

 

F23 with over 100K miles?
When was the last time the valves were adjusted?

 

Yes it is and its over 180, 000. Bought It two years ago. Not sure if the valves were ever adjusted. Most likely not.

 

Check the valve clearances.
Usually the exhaust are tight, if they are they may hold the valve open when hot causing low or no compression and a misfire.