so the gas hot water heater decides it no longer wants to provide hot water. Last time the pilot needed to be re-lit about 1 year ago. Water heater is of unknown age but guessing about 15 years old.

Pilot won't stay lit. Usually that is a thermocouple replacement so off to plumbing supply store.

They dont have that one in stock but suggests that it is actually the gas supply valve that is the issue. He suggests calling the 800 number on the side of the tank with the serial number to see what they say.

Call them, very nice very knowledgeable. After describing exactly what happens, pilot lights while holding down the reset button after 1 minute+ when releasing the reset button the pilot goes out.

They declare that it is the gas control valve.

Back to the plumbing supply store. They dont stock that part but can have it the next day. Price is $260, new gas hot water heater is $579. I hem and haw, roll the dice and order the part.

Pick up part the next day. Guy behind counter states make sure it is well drained before starting. I nod in his general direction, go home, hook up hose and drain valve doesn't drain a damn thing.

Much prodding and poking thru the sediment with a clothes hanger and a few experimental turn on cold water tap to blow water across the room I ensure that it is draining. Take a well deserved break to let it drain completely

Ensuring the gas is off then disconnect the main gas line to the valve remove the feeder in the bottom, the thermocouple and the other control line. Remove the coupler from the main in then the instructions say carefully insert a pipe threaded to the same diameter of the main gas inlet line and remove carefully rotating clock wise.

Not having such an item and the plumbing store counter laughed and stated beat it off with a hammer. Ok I can do that with one arm I think. (left handed Left wing badly damaged rotator cuff.)

Beat the living crap out of this thing for 20 minutes using a long extension for a ratchet it starts to move.

Pull it out and water starts gushing like it is Niagara falls. Screw it back in for a few threads. Apparently there was a lot more sediment than I thought clogging the lower drain.

get catch basin,
pull, plug, empty basin.
finally slows down enough to replace old with new.

get everything back up and snug, reconnect the gas, thermocouple and inlet. Test all connections for leaks, fill up the tank with water and let the faucets open to let out any air to bleed system.

set system to pilot, turn heater to lowest settings, ignite pilot wait the requisite 60 plus seconds. Second I let go of the reset switch pilot dies.

Fuck it going to home depot and buying a new one tomorrow.

that was a concern until wrestling the new one in place and accidentally bumped the gas supply on. Lots of gas quick. Turned it of quick evacuated a sensible mount of time. Taped the valve in the shut position so it could not be dislodged accidentally. It fired and worked as it should.

After some reading I decided benefit/cost wasn't there. Used to be paid eons ago to fix and install gas hot water heaters so was comfortable doing it myself. As far as the diags go, I will remember that a tank older than X is cheaper to replace than repair.