When your prints don't look right, it's tempting to blame the printer. But print quality issues usually have a specific cause, and figuring out which one saves you from replacing parts you don't need to replace.
Faded or light prints are the most common complaint. If the text is getting lighter across the whole page, your toner cartridge is probably running low. Remove it, give it a gentle side-to-side rock, and put it back in. This redistributes the remaining toner and can get you through another hundred pages or so. If that doesn't help, it's time for a new cartridge.
Streaks running down the page, usually vertical lines, point to something on the toner cartridge drum or inside the printer's imaging area. Try removing the cartridge, checking the green drum for scratches or debris, and gently wiping any dust off with a soft cloth. Don't touch the drum surface with your fingers. Oils from your skin can cause more streaks.
Smudgy prints where the toner rubs off when you touch the page usually mean the fuser isn't doing its job. The fuser applies heat to bond the toner to the paper. If it's worn or not reaching the right temperature, toner sits on the surface instead of melting into the paper. Running the printer's built-in cleaning cycle can sometimes help, but a worn fuser eventually needs to be replaced.
Ghosting, where you see a faint repeat of an image lower on the page, is another fuser-related issue. It happens when the fuser drum retains an image from one rotation and transfers it again on the next pass.