Many problems are solved by replacing thermocouple, but usually pilot will not light if thermocouple is bad. And some Reliance water heaters, the thermocouple is grouped with TCO and not generic part from hardware store.
If the combustion chamber is sealed with gasket, you might need spare
gasket on hand before taking out the burner assembly to replace
Gas water heaters can malfunction for many reasons.
Service tech is best resource unless you have manual and can take time learning parts and how to replace.
Gas water heaters require periodic cleaning of combustion parts and tubes.http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
Other ez problems include:
Gas shut off valve not fully open and handle is not parallel with gas line, so gas is partially shut off
If water heater flooded, then replace heater.
Excess water vapor from bad venting rusted out the burner parts. Inspect burner for rust.
Not enough combustion air, for example the house was recently
weather-tightened, or a louvered door replaced with solid door. Or air
intake screen located on bottom of heater is clogged with lint or dust.
Very hot attic can cause pilot to go out.
Trace amounts of pool chemical, laundry chemical, process chemicals, wax, etc can cause damage to burner parts.
Low gas pressure can cause problem.
Air in the gas line.
Bad gas control valve.
If ECO was tripped, there should be no gas to pilot.
Gas control valve might have debris or water from gas line.
If FVIR event tripped FV sensor, that would explain problem, and you would need service technician.
If repairs cost more than $330, then buy and install new water heater yourself, same day.http://waterheatertimer.org/How-to-install-gas-water-heater.html
Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.