Far North Scooby

Intermittent P0409 code. I have a recurring P0409 code that has cleared by itself on at least one occasion and when erased with an OBD2 reader stays off for around 10 to 20 engine starts. I've had the EGR valve cleaned by a local garage and that appears to be working fine but the fault code keeps coming back. It's been suggested to me that it could be caused by an intermittent open or short somewhere in the wiring harness possibly caused by the heavy salt corrosion where I live and could be something as simple as a corroded plug just needing a cleanup. Anyone come across this before or got any suggestions? I could take it to the main dealer but that's 50 miles and two ferries away and with an intermittent fault this could be very inconvenient if the fault is not corrected at first attempt.