Replying to Canon 40D

I could really do with your help! if you own a 40D could you switch to manual mode, set the appature to f/4 go to live view with exposure simulation turned on and decrease the shutter speed (lengthen the exposure) until the "exp.sim" indicator on the screen flashes, this is the point at which the sensor can no longer collect enough light to accurately display the predicted image, I have a horrid feeling mine is broken, after some idiot in the camera shop trying to sell me a new lens tried fixing a Nikon lens to my Canon body....

Much appreciated, +++Karma for taking the time to help me out.