state water heater model PR 6 75 NRVET Z 150 gallon side vented. No hot water. Ignitor glows, flame starts up and then shuts off 2 seconds later. Red blinking light comes on. Till now, helped by turning switch off and on. Now no go.
could it be the ignitor assembly? Is that difficult for homeowner to change?

Sounds like a thermostat... you need a gas fitter plumber probably unless you are handy that way