As answered so many times, the fix depends on lots of factors. Does it display it constantly? Are their actually any CDs inside the radio? Are you using original CDs or CD-R disks?
Do not use CD-R disks for a start, they are the main cause of most problems people have with this radio. The mechanism relies on the CD's being a certain thickness and cheap CD-R disks can jam the mechanism or jump out of the carriages.
Have you tried removing power for a minute and powering it back up? You will need your code but this makes the radio re-learn what disks are in the machine. It takes a minute and it makes some noises as the mechanism checks every disk location but this is normal. Sometimes if the mechanism thinks there is no disk in a position and there is one, it throws an Error3 up. Alternatively, you can open it up but unless you have instructions on carriage alignment and dissasembly, I would not advise because it is too easy to break it beyond repair.
Dave, May 2006