How to read engine codes,Windsor Monaco 40PST,ISL 370

On my 01 HR Imperial there is a rocker switch on the dash with all the other switches called diagnostic. With engine off turn on the key with this switch engaged and it will blink out a code in a series of blinks. I read a code on mine that turned out to be a fuel pump problem. I googled cummins codes and it showed me what it was. When I took it in to a cummins dealer they hooked up their computer to the diagnostic port under the steering column and read the same code.