My heater stopped working so I checked it, the light is blinking which means: Line voltage(115VAC) polarity reversed. If twinned, refer to twinning kit instructions.

I've read answers from in 2009, saying the flame sensor may need cleaning or that the panel is miss wired.
The heater was working fine before. Nobody touched the cables.

The way to verify if the polarity is correct is to measure from the incoming black to ground, then the incoming white to ground. You should get +/- 120 volts AC from black to ground & virtually nothing from white to ground.