This solution may possibly help fix Nokia 7230 if any certain keys stop responding or not working.
This solution shows the line paths of the Numeric keypads only, the navigation keys like SELECT, UP, DOWN, LEFT, RIGHT, Soft Left, and Soft Right and including the Power Key are connected within the Flex keypad membrane, so if the above keys only stop responding you may need to check and replace the flex cable first before using this solution below. Since all keys including the Upper keys or Navigation keys and the Numeric keys are both connected to the EMI-ESD filter IC. There is a certain point and possibility that all of these keys may not work or stop responding due to a water damaged, you may then proceed to use this solution below.
Nokia 7230 Keypad IC Jumper ways and line tracks
The numeric keys are group by outer and inner core layers of connections. There is a resistors which has a resistance value of 470 ohms that is being used in particular connections, you may check and replace this components also if found damaged.
The ESD-EMI filter chip protect the circuit from Electrostatic hazards, there is a possibility that this chip will breakdown and cause the keypad not to work or function, for it cuts off the its connection line paths through to the Application Processor Chip or CPU. Some factors are those due of water damaged, dirt, molds and oxidation build up.