I'm taking a look at the "Spectramax plus" instrument manual, shouldn't be much different from yours:http://bti.cornell.edu/manuals/Microplate_Manuals/Microplate_User_Manual.pdf
On page 46, it suggests that error 313 might be due to a problem in the electronic board, or to stray light coming to the detector.
Basically, the instrument is doing a reference check to see how much light it's getting. This is the check that's failing.
If it's "stray light", then you might have some strong light source near the reader that's interfering with the reading,
If it's the board, apart of trying to check if any cables are loose, I'm afraid it will have to be serviced.
There's no plate or protective sheet inside the reader at power-up, right?