If you only have another cartridge that is new in the box, open it and swap because it's cheaper than calling us for a service call to open it for you. You can always put it back in the box and use it later if it turns out that it isn't the cartridge. The above is intended to advise anyone who has a printer with a cartridge that has the drum in it. All Hewlett Packard, most Apple, all Canon, a good number of QMS, a few Brother machines and a good deal of other brand name printers use this type cartridge. The idea will above still applies.
Some printers have a separate drum and toner cartridge.
On these machines, the above will not work unless you change the drum unit too. If the above doesn't help, look at your fuser. This is the last major device that the paper passes through in your laser printer. It does this with heat. The fuser consists of two rollers one on top of the other.