I have a State brand power vented water heater model # PR675NRVETZ which shuts off every few days and it's red light blinks. Even when I flick the switch on the unit, it does not always help. Sometimes the igniter glows, the flame strs but then cuts out 20 seconds later. There is nothing blocking the vent outside the house.
Someone who looked at it was surprised at the heat generated by the flame. I don't know if that's a symptom or he was not accustomed to a unit with so much BTU.
Any help with a diag so we can change out the part, would be greatly appreciated.

Make sure your vent is not separated along the way. There are 2 vents. Smaller vent inside large vent. Smaller vent discharges fumes while the outer vent sucks in oxygen. Make sure both vents are fitted tightly into each other and secured by sheet metal screws. Separated vent will cause burnt gases to be sucked into oxygen intake, killing flame in the process.