If your miner is showing a lot of x's in a row and is hashing lower than 2TH/s it may indicate an issue with the hashing board.
If this is the case you can identify the specific board with the issue by testing each board individually.
You should first remove the top cover to the case, you will need to remove the Philips screws holding it in place.
Here is the Each Cable's Chain Number. Chain is defined by the Data Port on the Control PCB rather than the physical location of the S4 hashing PCB in the system.
You should then disconnect all but one data cable(there are 4 total) in order to test a single board.
Once all but one is disconnected.
You can power the miner back up(make sure the data cables don't touch the fans).
Your miner should now hash at about 500GH/s, you should then test the rest of the boards individually in order to determine which one is hashing low.
Once you have found the board causing the problem you can unscrew it by removing the screws holding it to the heatsinks.
You should then send the bad hashing board to the warranty center for replacement.