Heater fires up and sounds like it's running for a few seconds, it blows out heat, then it cycles and tries again. Had a pro out he swapped out the flame sensor. He said the ignitor and gas exchanger were fine. I got it to run one time after about 10 tries of it firing up and it ran the entire day and it hasn't worked since. Gas company checked the line said the gas pressure was fine. He couldn't get a hold of Hayward support and has gone radio silent since he was here. I can't get anyone else to come out everyone is booked. What should I check next?