I have a Carrier 58 MVP variable capacity condensing gas furnace. It stopped working. I called the service person and he replace the Flame roll out switch. The furnace worked for 1/2 an hour or so and it quit again. I called the service person and told him that the flame roll out switched is tripped again. He said do not touch any thing I think your heat exchanger is cracked and need replacement. How can I know for sure . Is there any thing else in the circuit causing blower fan not coming ON, overheating the heat exchanger causing the flame roll out switch to trip out. I am not too sure. Your help will be very much appreciated. Thank you

He said do not touch any thing I think your heat exchanger is cracked and need replacement.

A very good posibility it is defective. You will find out when he comes back.