Rebel XSi photos are consistently sharply focused on the right but blur on the left

[frugalfanatic]

I have cleaned the lens and mirror in an attempt to fix the problem but it still persist be it auto or manual focus.  Please help.  Thank you.

[TCampbell]

It sounds like either

(a) the lens is out of collimation (the optical elements in the lens are not aligned correctly) -- has this lens ever been dropped?

or 

(b) the sensor plane isn't level.  There are actually three adjustment screws which "shim" the sensor (both left/right as well as up/down).   The adjustment isn't user-accessible (the camera requires substantial disassembly to reach them.  It would have to be sent to service is this is the issue.)

Both of these require that either the lens or camera body need service.  Do you have another lens you can use to test.  If it does this with every lens, then it's the body.  if it does it only with one lens, then it's the lens.