I used to get error E-03 quite a lot.

When yo did the head change, was a cd still in the changer in the 'play' position?

I first encountered it when I was listening to cd, parked up, turned off car, but decided to eject cassette to retrieve cd's. I replaced empty cassette. When I re-started car, changer tried to load a cd which wasnt there, and jammed. Answer was to remove changer, take top off, and manually eject cassette, re-load and when changer back all was ok. Since then, have got E-03 about 5 times, each time was a disc which wouldnt return to the changer.

Fix was the same, remove changer, take of the top, thin screwdriver, gently coax cd back to cassette, and manually eject - there's a small arm at back of where cassette is, just push with screwdriver.

Mine now has error E-01, which I couldnt overcome, and had to replace the changer!!!