Dell 1320C printer - Feint printing but toner level is good

I have loved my dell 1320C for more years than I care to remember. I actually have three of them which i have butchered for parts to keep one good one running.

It has full toner cartridges, but it's printing feint (or invisible) black lines on maps I am printing and text etc is very feint too. Could someone please suggest what parts may need replacing or what the fault may be?

thanks