Diagnosis
If a new toner cartridge didn't fix it, the problem is in the imaging unit—specifically, the drum or the transfer roller inside. Toner isn't being properly transferred to the paper.
How to Fix It
The drum unit (Canon part 053) is a separate consumable from the toner. Open the front cover and you'll see the blue toner cartridge. The drum is the larger green unit that the toner snaps into. Pull the toner out, then lift the green handle on the drum unit and pull it straight out. Inspect the green photosensitive drum cylinder for scratches or wear. Replacing this drum unit usually clears up persistent faintness and streaks.
If That Didn't Work
The next suspect is the fuser assembly, but that's a bigger repair. If the streaks are smudgy or the toner rubs off, the fuser isn't heating properly to melt the toner onto the page.
Heads Up
Keep the new drum unit out of direct sunlight. The drum is light-sensitive, and exposure can permanently damage it before you even install it. Unpack it in a normally lit room and install it promptly.