Without the make/model number its difficult to troubleshoot.
Could be the probe not sending the signal back to the board..
If this is the case, that gives 2 options.. probe is bad/going bad.. or board is going bad
make sure the ignitor/probe is in the flame good. You can take any corrosion off with steel wool.
When the WH fires up, does it sound like a jet engine?
Is there any corrosion around the solenoid(s) on the gas line?
22 Year Air Force Civil Engineer, Retired.. Generator Tech at Grand Forks AFB, Part time RV Tech and Parts Manager at a Jayco Exclusive RV Dealership