Even if the fuel won't ignite, you should be able to get hot water and even heat by using the electric heating element.
We had our system give out last year, but managed with just the electric part for over a month, even in some pretty cold weather. Our problem turned out to be a bad sensor -- the one that tells the system if the fuel ignited. It was igniting, but the sensor didn't work so it kept telling the computer that there was no ignition, so the computer shut the system down.
2018 King Aire