I had  a Thermal Cutoff fail on mine Bounder 33C which did that in my case. It is a wire and a small diode encased in a small clear plastic tube, you will see it as soon as you remove your water heater panel. It is designed to blow if you water heater temp gets too high, like if you run both gas and electric at the same time. They are relatively cheap to buy and you should be able to visibly see that other one has been heated up or blown!

Good Luck!

Dan Quasnick