I recently purchased a a second hand Pioneer DEH 535R stereo for my
car and initially it was working. Now sometimes when I play a cd it
will stop and say 'ERROR RD', even though the cd is new or scratch
free. However now it refuses to accept the cd into the player,
altogether.

Because I do not have the manual I don't know whether I have pressed
something which has locked me out of the player so I can't play cds or
it is genuinely broken.

Has anyone else experienced this or have any idea what may be the
problem?