I have a Hayward H400 natural gas, electronic ignition from approx 2005-2006 model. It worked great for the first two years, then we had to replace the heat exchanger, once again worked great for a few years with no leaks. Last year it would light then shut off but generally it would eventually stay lit after a few tries, however by middle of the year say July, it wouldn't stay lit at all. I cleaned etc etc etc, eventually replaced the flame sensor ( even though the old was not damaged or rusted), this seemed to fix it, it generally worked fine the rest of the year. This year with the new flame sensor from last year, it simply won't stay lit. I have cleaned the jets that are right by the access panel but not the actual burner under the heat exchanger. Once again the thing will light and burn, then 5-8 seconds later it will turn off, you can hear the ignitor sparking almost the whole time it is lit even though it lights immediately. I inspected the flame sensor, it is 100% new looking with no defects or rust. Any suggestion? I really would rather not pay 300$ for a tech to come out and look at it. I do a lot of plumbing/ electrical work on diesel locomotives for a railroad so feel pretty comfortable about fixing it, I just don't know where to start.