A. Probing :-
May I know whether the burners are properly clean or not ?
May I know whether the burners are fitted properly ?
May I know is the flame is normal or high ?
B. Handling :-
If the burner are black due to usage:- Then denied politely to the customer that it may happen due to usage and close the call.
Troubleshooting steps-
Please clean the burners from the upper and lower sides properly. If you do not clean the gas stove glass & burner just after cooking food, you might face this issue. Always clean the burners properly after every use.
If the burners are black due to flame issue :- Assist customer with the tech assistance.