Terminator Aquastar 125FX Tankless Water Heater: FX Hood Horizontal Vent "pilot light goes burner cycles" Questions

...light unit is 5 yrs old. Turn on water 7 fan goes on, pilot lights for 2 seconds & goes out for 1 second & back on for 2 seconds & repeats the cycle but doesn't light. Some times turning faucet off & ...