This solution may help as a guide to repair certain keys that does not working when being pressed. If the device suffered wet or water damaged or any means of liquid most oftentimes that keypad may stop working due to some short circuit occurred and or oxidation may build up due to a liquid residues.
If this happens you may need to clean first the keypad membrane and contact pads using a liquid cleaner.
If all the keypad keys already been cleaned but still there are particular key may still not work, you can used the solution below to trace that particular and check if the connection line paths is not open or cut-off.
Nokia C5 keypad not responding - connection tracing repair solution
There are some factors that this keypad filter IC will breakdown or become faulty due to excessive electrostatic discharge and electromagnetic interference.
Hints for troubleshooting Nokia C5 keypad malfunction:
a) Clean the keypad membrane and contact pads.
b) Check the keypad flex wire if damaged, you may need to replaced the whole keypad flex matrix if the flex is already damaged.
c) Identify which particular keys were not working, then trace its connection between the other keys. If all of the keys connected in the same line paths are not working and the flex is okay, you may then suspects the problem lies on the keypad filter IC.
d) Replace the keypad filter IC.