wont do any harm but wont prove anything either, I was always told that you cant rely on FGA test on a boiler with a cast iron heat exchanger that boiler must be inspected, Like lots of things Fireman just because BG decide its OK doesnt mean that it is , I know of a case where a BG operative stuck a FGA probe up a baxi fire and BBU flue and told the customer that it had 17 PPM CO so she needed a new boiler and he would get a salesman to call, only came to light when the elderly customers son called Baxi to ask if this was correct