My C550 is leaving a broad 2-3 inch slightly lighter streak across the middle of dark solid colour areas, This occurs when we try to print a solid clour for a brochure on A3 for example. Any ideas on

THE PAPER SELECT ARE NOT COMPATABLE, SELECT LGL IS EQUAL TO 8.5/14 FLSC IS EQUAL TO 8.5 /13. TRY TO SELECT BLACK COLOR OR CLEAN YOUR DEVELOPING UNIT.

Check and clean mirrors.
also check toner.
its can be bad drum cleaning or developer.

change the roller

On the screen you need to scroll to troubleshooting and then to eliminate light stripes, and eliminate smears. Run those a couple of times each.
Then check the maintenance kit.

A technique to improve print quality is to try and clean the cartridge nozzles. You may clean the cartridge by following the steps given below: 
1. Go to the "Lexmark solution center." This can be done by clicking on "Start," go to "Programs" and Lexmark printer group. Click on the "Lexmark Solution center." Next click on the "Maintenance" Tab.  2. Select "Clean and Fix Horizontal streaks."  Click on "Print". This will produce a printed page. 
The colors that you should see here are black, yellow, magenta (a reddish purple) and cyan (a dark sky blue). You should notice the diagonal lines above and below the solid horizontal lines. These diagonal lines should be solid from left to right with no gaps. 
If there are gaps in the lines or the print diminishes across the page, you should run the "Clean Print Nozzle" page a couple more times to try and clear it up. If you still have gaps at the top and bottom, you should "wick" the cartridges by following the steps given below: 
1. Please open the "Solution center" again and go to the "Maintenance" Tab.  2. Select "Install color/black cartridge" and remove the cartridge or cartridges from the printer.  3. Get a damp paper towel.  4. Set the ink cartridge right side up on the damp paper towel so that the ink nozzles are against the towel and hold the cartridge in this position for about 15 seconds.  5. Now wipe the print head (on the bottom where the ink comes out) across in one direction only (left to right) until it looks clean.  6. Next dab the cartridge on the towel to make sure that the ink is flowing.  7. Snap the cartridges back into the printer.  8. On your computer, select "Old Cartridge install."  9. Repeat the "Clean Nozzle Page" one more time. If this fails to address the problem the entire procedure may need repeating.

Your yellow problem sounds like a bad yellow developer housing. Take the drum out and look at the developer housing, You will need a flash light to do so. See if the roller is solid yellow. Even if it is there may not be any developer left in the housing.
The Cyan sounds like the drum since it gets worse as it prints.
That can also be caused by the transfer belt cleaner. But that normally is the opposite. It has a rainbow sort of streak and it gets better as the machine runs.

tthanks about that but i dont know how to deel with that

If the issue is occuring in the same place on all prints, try the imaging drum (can't remember if this printer even uses one) or the transfer belt/roller (not sure which it uses). If you phone Dell support, they should run you through troubleshooting and send you either of those parts free of charge if you are under warranty. If not, buy them anyways; they don't degrade over time (if kept in the dark) and can be saved for when the currently installed one runs out if you don't need it to fix this issue.

Shake the toner cartridge left to right, then try.

Hi. the problem is the print head, maybe is clogged or damage, try this remove the cartridges , and the print head and submerge the print head in hot water for 3 minutes, dry the print head and reinstall it, if problem persist replace the print head.

TCT How to remove and clean Canon Printhead