I was quoted over $1,000 to remove the heat exchangers to check for a cracked one. SoCal Gas service teck said, that while removing the heat exchanges was the most accurate way to check for a leak, there are a lot of others that could be employed that would cost very little, or at least a lot less. He fired up the furnace for quite a while and checked several registers. He found zero evidence of a CO leak. Brandon only check one (which was also checked by the gas company guy).