If you try to fix firmware problem, simply flash the phone to it's original firmware version. The firmware version for Nokia C2-03 is RM-702.
If you do have hardware problem, follow the simple hints below.
1) Check the flex and LCD pin to pin connections, ensure that it is properly in good contact. A weak or loosen contacts will cause the display problem.
2) Check the LCD screen for possible damaged or cracked. If you ain't sure better have a extra LCD at hand just for a quick test.
3) Check the flex ribbon cable for possible damaged. Since Nokia C2-03 is a slider phone, the possibility of display problem is cause by flex failure.
4) If both the flex and LCD were already confirmed working and okay. You may then proceed in troubleshooting certain components on the Nokia C2-03 PCB board.
5) See the solution below, check the highlighted components surrounding, clean it if there is an oxidation spotted.
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