The "PCI Bridge Failure" error on the Lexmark 4600 indicates that the scanner has encountered a problem communicating with the printer.

Step	Action	Image
1	Power the scanner and printer off for 20 seconds.  Power the printer back on, and then power the scanner back on. Did the message return? Yes: Continue to step 2. No: Attempt to print and scan.  If the error does not return, the issue is resolved.	N/A
2	Power the scanner and printer off.  Remove the PCI bridge cable from the back of the scanner and the back of the printer.	PCI Bridge Cable click here to enlarge
3	Look at the connector on either end of the PCI bridge cable.  Are any of the pins bent, broken or missing? Yes: Continue to 'Still need help?' below. No: Continue to step 4.	N/A
4	Reinsert the PCI bridge cable into the printer and scanner.  NOTE: When installing the cable, be careful to ensure that the connector is in the correct orientation. The cable has a D-shaped connector (see image) that matches the D-shaped connector on the back of the scanner and the printer.  It is nevertheless possible to insert the connector in the wrong orientation and force it into place.  If you have a newer version of the cable, the screw standoffs may be color coordinated so that they match a sticker on the new PCI cards.	PCI Cable Connector click here to enlarge PCI Card Connector click here to enlarge New PCI Card click here to enlarge
5	Power the printer back on, and then power the scanner back on. Did the error return? Yes: Continue to step 2. No: Attempt to print and scan.  If the error does not return, the issue is resolved.	N/A

Contact Lexmark Technical Support for additional assistance.  When calling for service you will need to know your printer model type and serial number. 
Please call from near the printer in case the technician asks you to perform a task involving the printer.