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Do you have tank or tankless water heater?
What brand heater and what model shows on label?
If tank water heater, what does gas control valve look like? Is it electronic?
Open following link to help identify tank-type gas valve.http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
Does pilot light start up and not stay lit, or is there no flame whatsoever?
What color is the flame?
Are you using piezo igniter or a match to light pilot?
Is there a power vent sitting on top of unit?
What happens when you get more fresh air to unit?
1) Maybe the thermocouple is bad. Or thermocouple is mis-aligned and not sitting in the flame.
2) Probably the technician should have cleaned the combustion parts.
3) Tank-type and tankless water heaters require cleaning on a yearly or semi-yearly basis. This action prevents lighting problems, and help prevent ECO events and that require replacing gas control valve.
Cleaning and general service including cleaning burner, cleaning air intake if that applies, checking for proper venting and vent obstructions, adequate air supply, backdraft, testing thermocouple etc.
4) IF you have tank-type water heater, and do not have electronic gas valve or power vented water heater, the following troubleshoot walk-thru is very good:http://waterheatertimer.org/pdf/Pilot-outage-troubleshooting.pdf
Example photo showing parts for gas control valve.http://waterheatertimer.org/images/IMG_9146-gas-control-thermo.jpg
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