I apologize for the delay in responding to your inquiry. I have researched this matter for a number of hours, and then typed a fairly lengthy solution. For some reason, just before it was to be posted it disappeared. (We have to love these PC's, right?)
There are approximately a dozen specific models of the 1100. As you haven't indicated any specifics asde from 1100, and based on your description, I have worked with the understanding that the machine is an 1100. Most often, light printing is caused by issues with the cartridge, drum, or photosensors, as relevant, in addition to printer parts not being clean. These matters become nearly mute points, however, as you have indicated that the printing of the test page was approprate. It is important to remember, though, that when you put a new cartridge in, the toner should be distributed evenly by shaking gently left to right / side to side.
There are driver updates available for this model. Assuming an OS of XP, please see http://h20000.www2.hp.com/bizsupport/TechSupport/SoftwareIndex.jsp?lang=en&cc=us&prodNameId=14925&prodTypeId=18972&prodSeriesId=25470&swLang=8&taskId=135&swEnvOID=228
The last point & print bundle prior to this, on the product-specific page, was dated in 2002. Reference: http://h20000.www2.hp.com/bizsupport/TechSupport/SoftwareIndex.jsp?lang=en&cc=us&prodNameId=14925&prodTypeId=18972&prodSeriesId=25470&swLang=8&taskId=135&swEnvOID=228
On this same page, you will find two tools at the top which could be quite helpful in evaluating the problem with your printer, under "Useful Links" - check to see if you need updated drivers on your system, and the second to help you locate the proper driver.
Have you atttempted to print either (1) from another application on the same PC or (2) by attaching the printer to another PC to test the print in this manner? Finally, "Fast" or "Economode" is a printing method which uses less ink and prints more quickly, however, it also then generally results in lighter output than a normal or fine print. Test pages are typically printed in fine mode, It would be fruitful to make sure that the printing preferences are set to normal or fine rather than economode, due to the need to determine the actual reason for your problem. I would also add that the problem appears to be between the printer and the PC, rather than strictly with the printer, given the status of the test page. Testing the printer with another PC may be one of the most valuable troubleshooting steps, and if there is still a problem, please switch the USB port, and the USB cable, with which the printer is communicating, in order to further eliminate these issues.
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