If the SERIAL NUMBER is within 510K to 906M (Oct 2005 thru June 2009)---the MAIN BOARD is the subject of a safety recall.
The original board has TYCO relays (red color).
The revised board has *black* relays.
The reason for this recall is that---the surface burners *could* remain on even if the control/touchpad were turned off.
Have your customer contact LG for a free replacement (if within the Serial Number range).
If the LG range is *not* part of the recall...
Remove the back service panel (top cover) and disconnect/reconnect/lock both ribbon connectors on the MAIN BOARD.
Occassionally---a customer may complain that either the left bank or right bank of surface burners is inop or works erratically.
This is caused by a poor connection (oxidation) of the ribbon connectors.