The heater temp lights up but the heater never turns on

You need too hit the temp button increase temp then hit enter or set

leave the heater unplugged in the aquarium in for several hours.Then plug it in briefly and see if the neon bulb lights up (sort of an orange color). If it is on, turn the temp set knob counter clockwise until it goes out. Then turn the knob clockwise slowly until the light comes on again. Now your heater is set for approximately the current temp of the aquarium water. Now adjust the knob in very small increments, waiting hours between adjustments until the water is at the temp you want.

What exactly is the heater doing. You turn on the filter pump, flip the switch on the heater, does it turn on? Does the board light up?

You need a new gas control valve.

Bad heating element. Shut the power to the breaker off, disconnect the wires off of the heating element and then check for (ohms) continuity across the two element leads. If it show OL on the meter then you here is an open in the element and it will need to be replaced. Good luck.

Probably a relay.

1. Turn off power vent blower and set water heater temp to vacation (lowest temp)
2. Turn on power vent blower
3. Turn the water heater temp from vacation (lowest temp) to hot (highest temp) four times.
4. Set the water heater temp to normal

lucky for you I jst did one of these the other day same problem,, check the wires on the ignitor, some of them are thinly coated and causes them to short out, wrap electrci tape around them individuly turn off the unit , wait 30 sec, turn it back on at it;s lowest tem hold both temp buttons at the same time till lights come on the after it lights set the temp