I just changed a thermocouple on a 40 gallon AO Smith hot water tank. The pilot is still going out after being left on for about an hour what else can my problem be if not the thermocouple

Hi farb60 and welcome, post what model heater you have for the best help. Is this a FLiR? Are the screen filters clean?

My hot water tank is a 40 gallon AO Smith its an older one it's been in here at least 25 years is it possible that the thermostat is turned up too high with this cause my problem

My hot water tank is a 40 gallon AO Smith its an older one it's been in here at least 25 years is it possible that the thermostat is turned up too high with this cause my problem

Make sure the pilot flame is completely wrapping the upper 1/3 of the thermocouple.