A while back I had the same thing happened to me, after I replaced the plugs. When I swapped the plugs on 2 or 3 plugs the rubber seal gasket inside the plug terminal was not seated properly and allowed water to get into the connection and foul the ignition. Take apart the plug for that one side and inspect the inside the terminal plug.

You also may have a broken wire on that harness, what you can possibly do is unplug each plug with a test light test the voltage on the plugs. If your not getting voltage I then would trace up the wire until you find the break.

 

i work with gs 300 . when cold start car , misfire occur and rpm jump up , down. then p0300 fault code displayed

 

 

i replaced my spark plugs on my 06 gs300 awd. I immediately got a misfire code on cylinder 3 and 4. then I suddenly started getting misfire codes on all 6 cylinders. I think I just got ripped off when I ordered my spark plugs. so I just purchased some that were recommended from one of the threads. hopefully, this fixes this massive issue.