Tried the following fix and seems to have worked.
I was able to make U052 go away by letting the printer find no print head on reboot.
1. Remove ink cartridges and print head
2. Power down printer for 3 min
3. Power up, let it make error “no print head”
4. Insert the same print head and ink cartridges
5. Begin regular cleaning cycle from windows-control panel-devices and printers-canon-maintenance etc. while printer is inspecting the “new” print head.
Note: my MP970 is still clogged so the problem might reappear after cleaning as it did before. But at least I can now scan.
Source: http://fix-your-printer.blogspot.sg/2010/11/error-u052-on-canon-printers.html