To print something, the printer spreads toner onto the right places on the page, then heats up the fuser roller to just short of the ignition point of paper (Fahrenheit 451) and fuses the toner to the page. That makes the inside hot, so the fan turns on.
My 4/600
Always turns the fan on when it starts to print something, and usually does not turn it off for a minute or so after the last page has printed (unless a page has lots of graphics and takes a while to image and the printer cools off enough). If yours prints a page and you cannot hear the fan come on, the problem is obvious -- the fan died.
You can probably just swap the fan from your "parts" printer, but if you did not have that, there are two other ways to proceed.
1) Read the label off the old fan. Look it up online, and often you can find out the air moved in cfm and the noise generated in dB. That actual fan is usually only sold in the handy 10,000-pack, but a fan the same size with a comparable air movement and similar noise rating can usually be found to do the trick.
2) Post a message on Fixyourownprinter.com with what part you need, and Moe or someone else will send you a quote on a used part from their "junkyard". Prices have seemed very reasonable when i bought misc parts from them before.
