My water heater is not heating water. It is in "Vacation" mode. I shut off the unit, and reset the temperature. The blower kicks on and then the propane fires up. It runs for over a minute and shuts off. Thermostat kicks it back on immediately, but it shuts down within seconds and repeats 3 or 4 times before going in "Vacation" mode once more and shutting down for good.

What is it?

Thanks in advance