The first thing I would check is the flow switch or pressure switch to be sure the movement of water is closing it as it should. If it remains open or doesn't close, the heater will not operate, thus no voltage to the heater. The flashing is courious, I would suggest terminate the power testing your GFCI, and then, reboot.
The other 2 components, once you verify circ pump is moving water, would be the temp sensor, and the hi-limit sensor you have already replaced and is a likely suspect. That sensor is measuring the temp of the water to match the set temp, or call for heat, as needed. The hi-limit measures the water at the heater, and is not a likely suspect.