Close cover 310: cover open, but it isn't - OKI C3400n Laser Printer

Take a look at this and see if this is why:

Open the front door, as when you have a paper jam.
On the left is about a 3" curved white plastic swing hing guide piece, slotted in the middle.
On the metal wall to the left of it is a tab bent out to fit into this groove. If the Tab is not fully in the groove, it will prevent the front door from "fully" closing on this side, preventing it from actuating the switch mentioned by others.

A simple fix that solved it for my OKI C5650 "310/311:Cover Open"

Hello,

had the same problem. It is easily fixed.
On my printer a OKI C5750 is had to main covers, at the front (which you have to open to fix a paper jam) and a big cover on top (which you need to open to replace toners).
On the left is a piece of plastic which (when closed) presses on a switch.
When you open both covers, and close the top one, you can see it on the left when it comes down.

You can easily remove the error (only for a brief moment) by prushing the shining metal switch to the back.

But the problem is that one of the rollers is in a wrong position.
So just take a flat screwdriver and press on the bottom roller, and it will click and the problem will be gone when you close the covers.