I have an old HP Laserjet 2300dn printer which recently started to print very light print. I tried all different settings but there is not much difference. Is it likely that the toner is faulty and that buying a new one will solve this issue? Or should I change the whole printer? This question actually has some sense, because toner is quite expensive comparing to used laserjet printers on eBay.
(Making general assumptions here, never used that actual printeR)
There is no real way of knowing without seeing the printer.
You say old - I saw a similar issue with a old HP Laserjet, I replaced the fuser unit and it made the toner stick better and a lot darker.
However, it could just be the toner as you said, but again, without seeing, there is no real way of knowing.
One quick possible easy fix if the toner is running low is to shake the toner cartridge some. Be careful though as it may release some toner while doing this. If it is just low toner, this might give you a few extra prints before having to replace it.
Like wil said, there not too much else you can change out cheaply. If you find a better deal on another used printer, that might be a better route if you don't want to try a new toner cartridge.