Over the past month we have noticed that the red colour on the left hand side of pages printed by our Color LaserJet 1600 has not been printing very well. I assumed that this was just due to a faulty cartridge that was not feeding toner to the left hand of the page. A replacement toner did not fix the problem so after a bit of Googling it turns out that this is a common fault with the CLJ 1600/2600 series of printers.
It seems that the physical layout of the printer is such that the fan has a tendancy to blow dust onto the laser/mirror housing and the red colour optics are lowest in the image so they collect dust most noticably.
I found a couple of suggestions giving details on the best way to fix this but found a document from Don Thompson to be the most useful. It gives a step by step guide as to how to take the printer apart, clean the optics and put it all back together again. It worked first time for me although I did forget to reconnect one of the cables so my first poweron complained of a fault. Once I had reconnected that then all works fine.