Check battery?

I am thinking it is the control unit on the furnace that is intermittent . A flame sensor error is telling you that the flame sensor is not working correctly. This may sound a littl too simple but make sure the wiring from the thermostat is good at the furnace and take a look at the fmale sensor also

I am having the same problem here. I cleaned the sensor and even had it changed. Still cold in the morning. Put new batteries in the thermostat and still isn't working properly.