[kamsoni]

"Light" cycle on dishwasher does not work.

[denman]

Here are your parts
Replacement parts for MAYTAG MDB7709AWW2 UNDERCOUNTER DISHWASHER | AppliancePartsPros.com

Check under the unit for the tech sheet.m Often it is tapes to the back of the kick plate.
If upou cannot find it let me know and I will try to get a copy for you.

The problem could be the control panel (keypad) or the control board.
In the tech sheet you will see a ribbon cable pin output list.
each ribbon cable track is common to several keys so see it the keys that use the same tracks as the "Light" cycle are working.

The usual way to trouble shoot this is to check the keys (Ribbon cable) with a meter. If the keypad checks out OK then replace the control board.
Still a bit of a crap shoot but your odds are better.
If you see that the keys on a certain track do not work you may want to try the following.
Connector Cleaning / Checking
Unplug the unit
Open it up
Remove the ribbon cable from the keypad to the control board at the control board.
Clean the edge connector.
If it is the type where there is not a connector just the insulation is removed, it can be cleaned with a pencil eraser. Do not use an ink eraser as it is too abrasive.
Check the tracks in the ribbon cable for hairline cracks. A magnifying glass helps here.
Would not hurt to clean the connector on the board. Use electronics contact cleaner or isopropyl alcohol and a tooth brush. Be gentle and do not reconnect till all the solvent evaporates.
Reconnect everything and give it a try.