The connections on the back of the heater control box are shown below.
Back (top two pictures) and front (bottom picture) of heater control box
If the Temp.1 thermal limit is exceeded then both the Temp 1 limit and control lights will go out and Heater 1 will switch off.
The temp. 1, temp. 2 and temp. 3 control lights are on when the heaters are being brought up to temperature, and flicker on and off when the correct temperature has been reached.