The 'fault' light is on because it failed to light off main flame and prove main flame lit.
Reset by turning ON/OFF switch off wait 5 seconds then back on. Water heater will attempt to light off.
Watch it from outside to see how far it gets in sequence.
Does it 'spark'? Strong spark?
Does main flame light? Does it go right out?
Do you have AC power source available? Have you tried it on electric?
Check that ECO hasn't tripped and is reset......
Two sets of t-stats. Located in outside compartment. Black cover. Left one is for 120V AC electric element. Right one is for propane. Push/reset.
Post what is does and what it doesn't do.