I happen to deal with alot of plumbing firms in my work. If you can, let me know what firm you work for.
: The thermocouple is usually the culprit. Sometimes the main burner needs to be removed from the heater to fascilitate replacing the thermocouple. You would need to shut off the gas supply, disconnect the pilot tube, main burner tube and the thermocouple to remove the main burner. Use the shortest thermocouple that will work and make sure you check your connections for leaks with soapy water.

: : Recently we have been experiencing a problem with the pilot light going out on our water heater. I assume the obvious place to start is replacing the thermocouple but I am not sure whether that is a DYI project or would best be handled by a licensed plumber. I am fairly handy and the job doesnt seem too difficult but thought it would be best to check with you first. If it is possible for me to handle this, I would also appreciate any advice on how to go about it and if there is anything especially difficult about any part of the job-thanks