Hello and welcome to Just Answer. I appreciate the question.
This is a fuser motor error. Turn off the printer, then reseat the fuser assembly. If the same problem is experienced the problem may be with these parts in this order.
If you need more detailed removal and replacement procedures, please let me know.
Have you replaced the fuser motor?
No. Is that part of the removable assembly?
No It's not. The troubleshooting procedures call for the replacement of the fuser motor first, but if it were on My bench I would try a fuser first (RG5-7602). That's the removable part in the back. If you want to replace the fuser motor (RH7-1628-000CN) the replacement procedures can be found on page 110 of this PDF document http://www.partshere.com/Models/Q3948A/CLJ_2820_Removal_replacement.pdf
The DC controller (RG5-7609-030CN)is also a possibility ( but not probable). The procedures for replacement of this can also be found in the above document. Look over the replacement procedures and see if you feel comfortable. If you do great. If not a repair shop should charge about $60.00 per hour.
Please let me know if this helps.