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I'd try taking the motor out and checking it with no load (or a very light load). It does seem surprising as servo motors typically have three cables - two for power and one for signal, so if you can steer right and return to centre, then all three must be working OK. Which possible means something inside the motor is jammed to prevent turning past a certain angle.
PS. I've just had a look at mine, it is only a -90 to +90 degree range, not the full -180 to +180 like some other servo motors. Also if you look really closely there are two small dots on the orange axle connector from which you can tell if it is rotating correctly, even if you don't add an axle with a bar attached. I had never noticed them before.
But I've realised that if you don't put the IR receiver between them, the dots misalign.