Or (depending on the scanner) it could just mean it was trying to hit an address which didn't respond (perhaps not because it's faulty, more likely that it doesn't exist).
65535 is just like a 'one size fits all' errorlevel - converted to hex it's FFFF (F'in Fing F'in F'ed)...
As Welly said, more info would probably be useful - does your scanner indicate which address it's scanning when that is the response?
Oh, and do you have an aftermarket stereo installed? There's a chance a 'generic' reader might throw that when there's +12V on the K-Line (maybe?)
All the best,
Rob.